Identification of Common Rocks PDF - Province of British Columbia

I Province of British Columbia I Ministry of Energy, Transport and Communications The Identification of Common Rocks by OFFICERS OF THE DEPARTMENT Reprinted 1980  TABLE -OF CONT...

I Province of British Columbia I Ministry of Energy, Transport and Communications The Identification of Common Rocks by OFFICERS OF THE DEPARTMENT Reprinted 1980  TABLE -OF CONTENTS PAGE Introduction 5 What Are Rocks? 5 Basis of Rock Classification 6 _____________________________ Sedimentary 12 Metamorphic Rocks 14  The Identification of Common Rocks INTRODUCTION plexity of its geology and the variety of its rocks, as to both character and age. Rock is the most abundant material of the Rock identification, to the advanced stu- earth’s surface, yet very few people can rec- dent o r petrologist, can be very involved, ognize accurately the common varieties of but it is the intention here to present only rock. This is because some rocks cannot be the simple essentials that may be understood told without close scrutiny and not many by anyone with the simplest equipment. people have the need or desire t o do so. Given a hammer with which to break off It is also because a single variety of rock pieces of rock or to reduce them in size, all may vary widely in appearance and may that is needed is a pocket knife, a hand lens grade imperceptibly into other varieties, and or reading glass of six to ten power magni- consequently rock determination is in some fication, and a dropper bottle of dilute instances too difficult for those who are not hydrochloric (muriatic) acid; five parts of specially trained. Most text-books, being concentrated acid in 100 parts of water. written for geologists who are of course Before starting to identify rocks it is ad- specialists, are too involved for those who visable to know something of what rocks are only casually interested. This booklet are, how they are formed, and the principles tries to make identification of the most com- of classification. mon rocks both interesting and easy for the ordinary person. The Department of Mines and Petroleum What Are Rocks? Resources receives many inquiries regarding With few exceptions, rocks are aggregates rock identification. Sqme want a name for of mineral grains, sometimes of a single a stone that has taken their fancy, some are mineral but more often of several minerals. “ rock hounds ” and mineral collectors, some They possess coherence under ordinary con- are prospectors, some are engineers who are ditions. Thus sand is not a rock because it uncertain of their nomenclature, and others has no coherence, and mud is not a rock are just people who are generally interested because it disintegrates if immersed in water. in nature. To all of these, text-books are The mineral grains may be held together by unsatisfactory, requiring more study than is a cementing material, as are the sand grains desired, and even most handbooks and de- in sandstone, or they may be firmly held terminative tables fail in general to satisfy together by crystalline bonds, as in quartzite any but the most serious. Most peop!e must or granite. One special case is a conglom- start from scratch, for although they prob- erate, which is made up of pebbles which ably have been taught some rock.names in themselves are mineral aggregates; another school they have not been taught the prin- special case is obsidian, a volcanic glass that ciples of rock identification sufficiently well cooled from the molten condition too rapidly to apply them in later years. The following for crystals to form. treatment assumes little prior knowledge on Most minerals are solid crystalline sub- the part of the reader, and is addressed only stances. They are described on the basis of to those who wish to know the names of chemical composition and physical proper- some of the common rocks about them. ties. The study of minerals is termed min- This booklet is addressed to British Co- eralogy and much time can be devoted to it. lumbia readers and deals with British Colum- About 2,000. mineral species are recognized bia rocks, but that does not mean that the to-day, but fortunately for us the common rocks in this Province are unique or that the rocks contain relatively few minerals which principles of rock identification are not uni- are comparatively easy to identify. The versal. The counterpart of any rock found chemical composition is not determinable by in British Columbia may be found in many the ordinary person, and so the physical other parts of the world. What is character- characteristics are the ones we must use for istic of British Columbia is not a particular identification of m i n e r a 1 species. Chief assemblage of rocks but rather the com- among the physical characteristics are the 5  is a solid mass of partly formed interlocked nearer or onto the surface and the rest of crystals, some of which may show no crystal the molten magma solidifies so fast that no faces at all. more crystals or only very small ones can If magma solidifies deep within the earth, form. The resulting rock consists of large it cools so slowly that the crystals have time c r y s t a l s scattered through a fine-grained t o grow to a good size and the resulting rock groundmass and is called porphyry. On cer- is medium or coarse grained. This kind of tain rare occasions, and under special condi- rock is called intrusive or plutonic. On the tions, part of a magma may produce ex- other hand, if the molten magma pours out tremely large crystals which form bodies o n the surface of the earth, it cools quickly named pegmatites. and the crystals do not have much time to The variety of igneous rocks is large and grow, and so the rock thus formed is fine many schemes have been suggested for their grained. Such rocks are called extrusive or classification. While these schemes are simi- Groins cesiented. Groiris interlocked. volcanic rocks. Occasionally the magma A$ is As in granite. lar in a general way, they vary ir detail. cools so fast that n o crystals can form and This is mainly because igneous rocks grade it hardens as a natural glass. This is called from one type into another and the problem crystal form, the hardness, the colour and belongs. Each division has subdivis':ens or obsidian or volcanic glass. At times magma is to agree where to put the dividing points. lustre of the mineral, and the colour of the classes based upon various physical or chem- may start to cool deep in the earth and some Most classifications are based on the kinds powdered mineral (or streak). ical properties. It is the purpose of this crystals form - then the mass may move and amounts of minerals present in the rock. In some rocks the individual grains of the booklet to deal only with the principal classes minerals are recognizable. In the coarser of rock. Geology - that is the occurrence rocks the nature of the grains and their rela- and distribution of rocks in nature-is not tionship to one another can he seen. In the considered. ROCK IDENTIFICATION KEY finer rocks, in which the mineral grains are Identification of rocks depends upon man- The following key is designed to identify rocks from freshly broken clean surfaces. not recognizable, and the rock has a dull or ner of occurrence, uniformity, grain size, Much can he learned from rock surfaces that have long been exposed to the weather. but for stony appearance, the hardness and colour character of grains, hardness, manner of the present purpose weathered surfaces are not considered. are more important as a means of identifi- breaking, colour or hue, and behaviour with cation. weak acid. Determinations should be made To use the rock identification key. it is essmtial to begin at the top of the table. D o Some varieties of rock grade into one only on clean, freshly broken material. Rock not tcy to begin part way down. Examine the rock closely with the naked eye and first another. Thus there is every gradation from at the surface may be dirty, covered with decide which of the four descriptions fits it best-that is, is the rock glassy, fine grained a shale to a sandy shale to a shaly sandstone lichen, coated with some mineral substance, (granular appearance but individual grains not distinguished). mixed fine and coarse grained, to a sandstone. Similarly there is every decomposed by weathering, or in some way or medium to c o m e grained (individual grains readily distinguished)'? Having decided the gradation between, say, a granite and a gab- may have its true nature obscured. A fresh grain size, next follow down the arrows below the class into which you have placed your bro. In the science of petrology, precise surface can usually he obtained with a few rock. From here o n use a hand lens for examination. At each point where there is a definitions are made and rocks are firmly strokes of a prospector's pick, Identifica- division, check all possible choices and decide which choice most closely fits your specimen, classified in the laboratory as a result of tion of beach pebbles that are smoothed and rounded is not considered, because this book- then continue on down from there until you reach a rock name. Once you have decided o n careful microscopic examination, hut the a name, check the description of that rock ;n the text of the booklet to see if the detailed reader of this booklet can be only approxi- let deals specifically with fresh surfaces. mate. He can only apply what are called Many pebbles are relatively easy to identify description suits your specimen. If the text description does not fit your rock. go back to field terms, those that are based on identifi- without breaking, but many are extremely the table and check for alternative choices at places where vou may have been unsure of cation in the field only. Geologists in the difficult, and breaking usually destroys their your observations course of their work employ field terms until interest or charm. As a rule a pebble catches such time as laboratory study is desirable. the eye and is picked up because it is an object which shows some special effect of Basis of Rock Classification colour or pattern, due often to layering, veining, or alteration. Such pebbles are not Rocks, like all natural o r artificial sub- good examples of common rock types. stances, may he grouped according to various characteristics. The fundamental grouping i s IGNEOUS ROCKS on the basis of origin, according to the geo- logical conditions under which they were Igneous rocks form from molten matter formed. Rocks are thus igneous, sedimen- that is called magma. As magma cools the tary, or metamorphic, depending on whether chemical elements within it combine to form they crystallized from molten material (mag- minerals. These minerals tend to solidify in ma), formed as the result of the accumula- Rat-faced three-dimensional crystal shapes, tion of discrete sedimentary particles, or are and under ideal conditions each mineral the products of change (alteration or meta- forms a complete crystal such as is seen in morphism) of pre-existing rocks. Every rock displays and illustrations. Usually, however, has characteristics that are the result of the so many crystals form a t the same time in process of formation, and so determine in a magma that each interferes with the which of the three hasic divisions the rock growth of its neighbour and in the end there 6  ROCK SPECIMEN r----l LIGHT DARK , GRAINS NOT E A S I I Y DISTINGUISHED , , GRAINS EASILYIDISTINGUISHED , OBSIDIAN W I L L SCRATCH WILL NOT FIZZ WITH A C I D N O FIZZ iUARTZ KNIFE SCRATCV K N I F E MARBLE WITH A C I D DENSE AND BRITTLE G R A N U L A R AND TOUGH L A R G E DISTINCT CRYSTALS ROUNDED PEBBLES A N G U L A R FRAGMENTS I N F I N E GROUNDMASS I N FINE CEMENT I N FINE CEMENT CHERT r L I G H T COLOUR INTERMEDIATE rDARK PORPHYRY CONGLOMERATE BRECCIA ANDESITE BASALT GLASSY GRAINS DULL QUARTZITE FELSITE I i FIZZ WITH A C I D I , I N O FIZZ Y l T H ACID , NO LAYERS A VIGOROUSLY WITH DIFFICULTY O R ONLY W I T H ROCK POWDER SLIPPERY FEEL LI ME STONE DO LO M ITE DOES N O T SPLITS EASILY INTO GREEN TO B L A C K T H I N SMOOTH SHEETS SPLIT EASILY SLATE SHALE SERPENTINE I NO L A Y E R i O R BANDS , , , DISTINCT LAYERS OR B A N D S , GRAINS I N T E R L O C K ED ROUNDED G R A I N S CEMENTED TOGETHER GRAINS INTER LOC KE D GRAVEL S I Z E MICACEOUS GRANULAR CONGLOMERATE SPLITS EASILY DOES N O T ALONG LAYERING SPLIT EASILY SCHIST GNEISS LIGHT INTER M EDlATE DARK MOST GRAINS M A N Y DARK GABBRO L I G H T COLOUR GRAINS QUARTZ DlORlTE FELDSPAR Q U A R T Z AND FELDSPAR QUARTZ SYENITE GRANITE QUARTZITE 8 9 The Common Minerals DIFFERENCES BETWEEN QUARTZ AND FELDSPAR I N IGNEOUS ROCKS Intrusive or Plutonic Rocks Qxorrz Diorire.-Is the middle member of in Igneous Rocks an almost continuous series of rock types The intrusive rocks are homogeneous, that grades from granite through granodio- The common rock-forming minerals are medium- to coarse-grained granular rocks rite, quartz diorite, and diorite to gahbro. quartz, feldspar, mica, pyroxene, and amphi- glassy dull that grade in colour from light to dark. transparent The gradation involves decrease in quartz bole. Quartz and feldspar are generally light For the accompanying chart any rock in content, increase in dark mineral content, grey i h i t e. grey, red, pink, green coloured, mica may be dark or light, and the irregular shape rectangular shape which the individual grains are large enough and replacement of orthoclase feldspar by others are generally dark coloured. irregular - broken surface Rat broken surface to be distinguished readily by the naked eye plagioclase feldspar. The change in mineral is considered to he medium to coarse grained. Quartz. -This mineral is so hard that it Mica.-Like feldspar, mica is the name of content results in a gradual darkening of The light-coloured intrusive rocks are often colour from the light granite to the dark will easily scratch a knife blade. In igneous a group of very similar minerals. All are recognized by the fact that they peel off into lumped together as " granitic " rocks and gabbro. Because of the difficulty of accu- rocks, quartz is usually transparent or trans- extremely thin flat and smooth sheets or the dark ones as gabbroic " rocks. Tech- rately naming a hand specimen of one of the lucent and grey to smoky i n colour with a flakes because they have perfect cleavage in nical classifications are usually based o n the intermediate rocks, only the one name, definite glassy appearance. In large masses it i s more likely to be opaque and milky one direction. The smooth flat faces are proportions of light and dark minerals, the quartz diorite, is included in the accompany- white but still glassy. In the rock, quartz shiny in the rocks. T h e black mica, biotite, presence or absence of quartz, and the types ing determinative table. F a r this purpose grains are irregular in shape because crystal is most common in igneous rocks, but the and relative proportions of feldspars in the any medium- to dark-grey granular rock of white mica, muscovite, is sometimes found rock under examination. These details can medium to coarse grain sire which contains faces are rare and quartz does not break o r and is common in metamorphic rocks. only be determined by microscopic study, feldspar and abundant dark minerals plus cleave along flat planes to form cleavage and far that reason only a broad general field faces. Pyroxene.-Pyroxene is also the name of some recognizable quartz is classed as a a family of minerals of which the common- classification can be used for hand specimens. quartz diorite. This rock is abundant in Although quartz is a mineral, it is included British Columbia. est is augite. Augite is dark green to black Cranirr.--ls usually a light-colaured pink in the accompanying table for rock identifi- or grey medium- to coarse-grained rock. It Grunodiorire. - Is intermediate between and forms stubby thick crystals that have a cation because it is so common and is found square or rectangular cross-section. consists of interlocked grains of feldspar and granite and quartz diorite and may he difficult in large masses. Amphibole.-This, too, is a group name quartz with small amounts of dark minerals, to distinguish from either.^ I n granodiorite F e l d w r. - Two groups, orthoclase and for several minerals of which hornblende is generally biotite and (or) hornblende. Some- the feldrpar consists of nearly equal amounts plagioclase, make up a family of minerals the most common. Like augite, it is dark times the feldspar is red and the rock will of orthoclase and plagioclase, whereas in known as the feldspars. For the purposes of green to black but it forms needle-like or look darker than normal. True granite is granite orthoclase is more ahundant and in this booklet all will be considered as a single long slender crystals that have diamond- not common in British Columbia. quartz diorite plagioclase is more abundant. mineral. Feldspar is almost as hard as shaped cross-sections. Often it is difficult t o Srmirr.-Looks the same as granite except Granodiorite is common in British Columbia quartz but can be scratched by it. In the distinguish pyroxene from amphibole in a that very little or no quartz can be seen in it. but is not quite as abundant as quartz diorite. rock the grains are dull to opaque, with the hand specimen of rock. Interlocked crystals of feldspar with minor Dinrife.-Is intermediate between quartz The colour of a rock depends mainly on scattered grains of mica or hornblende make diorite and gabbro and may be difficult to general appearance of porcelain. The colour distinguish from either. It h a a " salt and may vary from red through pink to white the relative proportions of the light and dark UP the rock. Syenite is rare in British minerals. If most of the mineral grains are Columbia. pepper '' appearance and contains no quartz. and grey to green. Feldspar usually forms Diarite is not common in British Columbia. quartz and feldspar, the rack will have an Guhhro.-Is a dark medium- lo coarse- welldeveloped crystals and it also breaks or over-all light appearance. If most of the cleaves along flat cleavage faces. Conse- grained granular rock. One-half or more of mineral grains are dark min,erals, the rock the grains are dark minerals, usually pyrox- Extrusive or Volcanic Rodis quently, grains of this mineral normally have will have a n over-all dark appearance. A rock some straight edges and display flat rectangu- ene, and the rest are dark plagioclase feld- The extrusive igneous racks are usually in which the dark and light minerals are in lar faces on broken rock surfaces. Often two spar. Gabbro is not found in many British called h a s or volcanic racks. They are nearly equal proportions is classed as inter- faces meeting at right angles are visible. Columbia localities. composed of grains that are generally IW mediate in colour..,.. -... _. -~ - Quartz diorite. Felaite. Rornlt. Interrnedinte. Light. Llnrk. 10  O h r i r l i r r r i wirh r w w l w i d d l m c t u n - , Anypdnloidni I n w. series of parallel lines, each of which indi- formed from them. Shales are usually grey small to he seen by the naked eye hut which said to he amygdaloidal. Basalt is the vol- cates where a layer or bed was deposited on to black or brown, hut can be almost any are, nevertheless, large enough to give the canic equivalent of pahhro. It is a common top of the layer below. Often plants and COIOIIT. They are easily scratched with a rock a granular appearance. Porphyritic rock in British Columhia. animals or their remains drop into and he- knife. Bedding i 5 usually quite plain in wrictieq in which scattered large crystals And~siic.--ls the fine-grained or volcanic come buried in the sediments-these become shale exposures, and sometimes the beds are occur in a fine groundma5s are common. rock nearly equivalent t o diorite. For the fossil:. very thin. Shale ha\ B characteristic earthy Eatruiive rocks form a gradational series accompanying table all the fine-grained rocks Co,,plo,iieniie.-Lool\r like concrete. It odour when hreathed upon. I f struck with with the same mineral comporition as the of intermediate colour are termed andesite. consists of gravel t h a t has become cemented a hammer, the rock h r e a k into chunks and serie5 of the intrusive group, hut they are Andexitc i~ more common than felsite hut together to form a rcck. Sometimes there does not split into thin Rat sheets. Some finer grained. Because the mineral composi- rarer than basalt in British Columbia. is not much cementing matter visible. and shales contain a considerable amount of tion normally is indeterminable. in hand Oh.ridinn.-1s a glassy rock. It is usually a t other times there is considerable cement calcium carhonate and may effervesce or specimens, o n l y three names. felsite. andesite, dark and so can he distinguished from mixed with sand in between the pzhhlcs. I n fizz if tested with a drop of dilute hydra- and h m l t , respectively light, intermediate, quartz, which is normally white in large the latter cahe conglomerate can he dirtin- chlori; acid. and dark in coloiir, are used in the accom- m a s e s. It has a conchoidal fracture (see wished from porphyry because the large Linzr.sro,rc.-ln easily recognized bccause panying table. These rocks are tough t o illustration). grains are rounded pehhles, mherras in a it will fizz vigorously when a drop of dilute crack with a hammer and break to a rough porphyry the large grains w e minerals show- hydrochloric acid is put on it. Limestones fracture surface. SEDJMENTARY ROCKS ing crystal shapes. Many conglomerate are composed of the mineral calcite, which beds are so thick that the bedding planes is calcium rarhonate. Ordinary limestone Felsi!e.-Is a general name used for fine- Any rock exposed on the surface of the cannot he seen in B single outcrop. grained light-coloored igneous rocks. Of is ii fine-grained white to grey or even black earth is acted on by rain, air, temperature. Breccia (pranouncsd hrrtchia). -- Is a rock that can easily he scratched with a the many rock types grouped under this plantF, and animals. These agencies all tend term used to describe a rock that consists of name, rhyolite, the volcanic equivalent of knife. It commonly forms from matter pre- t o hreak up the rack mechanically. In addi- sharply angular fragment7 of rock held to- cipitated out of solution. Fossil shells are granite, is perhaps the most common. Fel- tion, water removes parts of the racks by gether by a fine-grained cement. Breccias site exposures sometimes show a series of dissolving them. Most of the fragments and common in limestone and sometimes are so are formed in many different ways. and al- numerous that they make up practically all more or less parallel hands of slightly dif- the dissolved material eventually are washed though few are actually sedimentary they are ferent colouis cnlled Row h a n k These of the rack. Rrdding lines can he seen into a ri\'er and carried downstream. When included here for convenience. The word sometimes, hut at other times the beds are hands can he mistaken for the bedding seen the river empties into a sea or lake, the bits means broken, and the angular and broken so thick that the lines are too far apart t o in sedimentary racks or the foliation planes of rock are dropped and the dissolved mate- nature of the fragments a s contrasted to show i n an exposure. found in metamorphic rocks. However, as rial precipitates oiit. Thcse materials settle rounded pebhles distinguishes a breccia from a rule Row bands are wavy and swirly rather Calcite i~ a very common mineral that down on the floor of the hody or water to a conglomerate. frequently forms tiny veinlets and stringers than nearly straight like the others. Felsites form layers of sediments. The coarser hits Sondsrone.-Conrists of grains of sand are not common in British Columhia. through and coatings on d l kinds of rock. of rock, now rounded from rolling and cemented together. Sandstone grades down Since any calcite will fizz with acid, when Busali.-Is a dark fine-grained tough rock. humping. are called gravel; the finer par- through fine grain sizes to mudstane and shale testing for limestone it is necessary t o put It is often porphyritic, vesicular, or amygda- ticles are called sand; and the very finest and upward through coarse sizes to can- the acid o n a freqh and clean surface of the loidal. When some h a s cool. the gases grains are called mud or clay. The precipi- glomerate. As with conglomerate. beds are rock to he tested and to look closely to see escaping from the magma leave small tated material forms a very fine-grained mud sometimes t m thick to he recognized in that the rack is fizzing 811 over and not just rounded holes in the rock-these holes are or ooze. Due to the pressure caused by the specimens or small outcrops. On occasions along cracks or in isolated spots. called vesicles and the rock is said to he weight of overlying layers and the deposition the cement of a sandstone may consist of Dolo,,,irr.-Usually looks exactly like vesicular. If, as is frequently the case. the of cementing materials from water, the sedi- calcite, in which caw when testcd with acid limestone, hut normally it will not fizz if a vesicles later become filled with mineral ments are compacted and the grains ce- effervescence would result. drop of cold dilute hydrochloric acid i F put matter, especially agate, the filled vesicle? mented together to form sedimentary rocks. Shn1e.-Looks just like hard mud or clay. on it. If dolomite i5 ground to a powder are then called amygdules and the lava is Sedimentary racks characteristically show a and so it should because it is the rock and then tested. or if hot or concentrated 12 13  Mnrbk. Schist. C,$Pi.VS. acid is used, there should he good effer- Rakes, or sheets. and is caused by the growth vescence. Dolomite often weathers to a of thin platy minerals, especially micas, with other minerals in parallel layers. Meta- tically it has well-developed colour banding ELEMENTARY BOOKS DEALING brown or reddish colour o n the surface, due to the concentration of light and dark whereas limestone turns light grey or white. morphic rocks may farm from igneous, sedi- minerals in alternate layers or streaks. WITH ROCKS Also, dolomite tends to weather to a rough mentary, or other metamorphic rocks. M n r h k - I s limestone that has recrystal- Gneiss splits better along the layers than The following list of books may prove and finely angular surface, whereas lime- across them, but the splitting is not easy and stone weathers smooth. lized due to metamorphism. It consists of useful to those who wish to pursue the sub- a medium- to coarse-grained mass of inter- the split faces are not very smooth. ject of rock identification further. (Prices Chert.-Is a very hard dense rock that locked crystals of calcite. A magnesium- Schi~r.--ls a foliated rock intermediate as at April 1, 1970.) can be any colour, but in British Columbia is most commonly dark. It can he distin- hearing limestone or dolomite will change between slate and gneiss. It splits along the PRICE into a magnesian or dolomitic marble. Or- foliation more easily than gneiss but not as 1. Elementary Geology Applied to guished from quartz by its appearance, dinary marble will fizz with dilute acid, as easily as slate and tends to form Rakes or Prospecting, which is waxy or horny and smooth rather will limestone. Dolomitic marble will only lens-shaped slabs rather than thin sheets. I. F. Walker, than glassy, It is brittle, and when struck fizz if it is powdered or put in hot or con- Mica and mica-like minerals are abundant British Columbia Department of with a hammer breaks with a shell-like con- chaidal fracture or shatters into slivers. centrated acid. Marble is easily scratched and easily recognized in schist, and the faces Mines. sl.w with a knife. of split pieces of the rock glisten with the 2. Rocks and Minerals, Chert consists of extremely fine-grained sili- Qtrrrrtrilc. - Is metamorphosed quartz flat cleavage faces of these minerals. Zim, Shaffer, and Perlman, ca, the same chemical composition as quartz. It may he found as layers or ribbons sepa- sandstone. Quartz cement binds the origi- Serpentine.--Ts an oily looking green to A Golden Nature Guide, rated by thin films of shale and as thin ir- nal sand grains together so tightly that the black fine-grained rock that can be scratched Simon and Schuster, New York. 1.35 regular bands and lenses in some limestones. resulting rock is extremely hard and tough. easily with a knife. In many places it is When the rock is broken, it breaks through 3. A Field Guide lo Rocks and Min- highly sheared and breaks into scaly frag- erals, cement and grains alike and not around the ments with smooth shiny faces and a slip- METAMORPHIC ROCKS grains as in sandstone. The broken faces pery feel. At thin edges the fragments tend F. H. Paugh, Houghton Mifflin Co., Boston. 5.95 When any rock is changed from its origi- have a glassy appearance. Quartzite can he to be translucent. Serpentine is actually a nal nature by the action of heat, pressure, mistaken for vein quartz or chert if the mineral name, and more correctly a rock 4. The Rock Book, or chemical action, it is said to be metamor- grains are not distinctly visible. Pure quartz- composed of serpentine should be called a C. L. and M. A. Fenton, phosed and is called a metamorphic rock. ite is rare, and most varieties contain some serpentinite. Soapstone is a rock composed Doubleday & Co., Inc., New York. 11.50 The changes that take place at the surface mica; there is every gradation between of the mineral talc and can be mistaken for 5. Rocks and Rock Minerals, of the earth due to the action of the weather quartzite and mica schist. serpentine. Soapstone, however, is usually Pirsson and Knopf, are called weathering and are not considered Shre.--ls a very fine-grained brittle rock light green to grey in C O ~ O U Tand soft enough John Wiley and Sons, Inc., New t o he metamorphic. During metamorphism that splits easily into thin smooth-faced lay- to be scratched with the fingernail. York. 8.95 one or more of three main changes may take ers or sheets. The mineral grains are too place. Sometimes the original minerals pres- small to be seen without a microscope. Most ent simply recrystallize lo form a coarser- commonly slates are dark grey or green to grained rock-the, change from limestone black, but they can be red or brown. They into marble is perhaps the best example of are metamorphosed shales. The term argil- this. At other times entirely new minerals lite is used far rocks in between shales and may farm in the rack-the formation of slates. Argillite is harder than shale hut garnet in schist is a good example of such will not split or cleave " into sheets as slate a change. The third main type of change is does. the development of foliation-this is a struc- Gneirr (pronounced nice).-Is a medium- ture that allows the rack t o split into leaves, to coarse-grained banded rock. Characteris- 14 I5 Queen’s Printer for British Columbia C Victoria. 1980