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Lls diagrammes suivants illustrent la mithode. 1 2 3 1 2 3 4 5 6 MICROCOPY RESOLUTION TEST CHART (ANSI and ISO TEST CHART No. 11 M -APPLIED IfVMGE Inc — '^ ('16) 288-5989 - Fo» [reprinted from the sixth report of the «.*.o-^*^' BUREAU OF MINKS, ONTARIO.] Rocks and Rock Structures — BV — WILLET G. iMILLER. prospectors' COURSE— KINGSTON SCHOOL OF MINING. KKl'klNTEl) HKOM TlIK SIXTH KKI'OkT Oh TIIK IIUREAT ()!■ Ml NES, ONTARIO.] Rocks and Rock Stri:ctiires MY WILI.I'T (;. MILLER. prospectors' course— KINGSTON SCHOOL OF MINIKl Rocks and I^ock Stru(ti;rhs. liv WILL I-: J- (.;. Miij.HR. According to the commonly accepted tlieory the earth and the other mem- K„ckH „f the bers of the solar system were at one time in a fused or molten condition ; hence *'*''''''' ""**• the first rocks formed on our globe (through the cooling of the molten ma-s) belonged to the class wiiich is known as the Igneoua or heat formed rocks, igneous hk^Jb These rocks are still being formed through the cooling of Quid matter which lises from depths in the interior of the earth through fissures in the crust, to or towards the surface. After the molten material had solidified at the earth's surface, it would be acted upon by the atmosphere, water and other agencies, and be broken down to a greater or less extent to form gravel, sand or clay, just as wo see masses of rock, such as clifls, being worn down by these agencies at the present day. The materials thus formed, which are fra^'mental in nature, together with rocks formed by the deposition of the remains of plants and animals, make up the gre. ;»r part of the class known as Aqueous or water- formed Aciueoua rocks. They ar( g.ven this name on account of water being the chief agent ''''■'**• in their formation. They are known also under the names sedimentary, frag mental and clastic. The sorting action of water on the loose material into which rock masses are broken may be seen along the shore of any lake. The coarsest material, gravel, is deposited near the shore, while the sand is deposited in layers in deeper water, and the finest material or clay is deposited still farther from the shore line. Through the effects of pressure and cement substances these loose products are solidified in time into beds or strata of conglomerate sandstone and shale. MetRmorphia roe 1(1. FnrniH of i^neoim rnokN VolcAnic rucks. Plutonic reoks. If thKHo HquoouH ro .ks ftro HubiHctfd to Rrontnr pr«msnrn ami hoaf, they lifcomo \vr compact ami timir charautcrH urn iiiuth alttTcd. They Ix'coine nioreorlRHs ciyBtHllino in appnarnnw and aro ihim known hh M«'tainorphic or altered rookH, or as the Crystalline sohiHtH, Wo thuHhavpthrrckindH.or ihrtf urcat cluasea of rockH,niakin>?upth('crust or outer part of the earth, viz., the ri^ncous, A(juoouh and Mitmnorphio lockg. Metiiniorphic rocks are alno formed from thone of the ijjneoiiH class through the agenoy of presHuro and chemical actio Thus, while some Knoiasesare known to lie iiltercd .khu-ous roi-ka, it has heen proveil that o'herg have b(cn fornu-d through the alteration or metiimoiphism of iu'iitoua rocka. The earth in conntantly losing heat and diminiahiiij;^ in tizo. The interior as it loses heat, tends to shrink away from the external cool and solid orust. This r;au8f 8 the crust ti hncomo folded and wrinkled, as the cooling goes on. In jilacoH the side or luteral preSHure on the folds beconu h so grent that they are cracked and broken acroHS. The (i;^8uret^ or cracks thus formed ia the crust sometimes reach to grout d( pths, so that the highly heated matter of the interior (inds a passage to the tui fa •(•. On cooling it gives rise to surface- formed igneoiiH rocks, or, as they are gener.illy called, volcanic rocks. Tho materirtl of which these are composed having hoen expoHcd at tho surface of the earth cools quickly, after tho manner of the molten material which is drawn from a furnace. Hence we find that volcanic rocks are often sing- like or s'^ssy in appearance. Through tho folding and rupturing of the rocks, tho crust has tisaurcs and cavities formed on its under surface. Into these spaces molten matter also makes its way, but here it loses its heat or cools slowly, and gives rise to rocks which are coarse grained and crystalline, and not glassy or slag-like in apptiarance. Igneous rocks of this class are called pi u tonic rocks, since they are formed deep down beneath the surface of the earth. There is also another class of igneous rocks, which is intermediate in character, or forms a connecting link between the volcanic and the plutonic classes. This class of igneous rocks is known as tho dike rocks. They • represent molten matter which has cooled in narrow fissures in the crust, and hence has lost haat through contact with the walls of the fi.saures more rapidly than did the matter which gave rise to the plutonic rocks, but not so rapidly as did the volcanic rocks. Dike rocks are usually much finer grained than those of the plutonic class, and eonio of the minerals of which they are composed often have a definite crystal outline, giving the rocka a porphyritio structure. They also differ from the volcanic rocks in not being glassy, and in other characteristica. Some of the fissures formed by the fracturing of the earth's crust do not extend down to the highly heated interior, and hence are not filled with molten material. In course of time however most of these fissures are tilled with mineral matter, which is deposited from solution in the waters which circulate through the crevices and openings in the rocks. The material thus I I I ('..poditHd f'.riiiH what has b^en oallf-d vein rookg. Tli.-f« iiiatpriiild filling fiiHureM aro what ia known as minimi voin». VVhilo ilik.-i nntl tiHauro vtinH ^''■"' ""'"' ar»i Hiinilar in I'orni, flu' luttr arH of aqueous origin, whiln tlin former are of igmons I'irrnalion. It i8 natural tlmf, th« oldi-r rouk^ should in nioRt ciikcs l.o nior.* l.rokfn and fnictuyed thmi tliH nowcr ; hmcA> vcinH ure nion? frrcjunnHy Ibuiid in thi'tn. Jn tho projcKs of folding, opcniiij{H ure niiidn l.otwfcn tho hodn of rocks, and niorcovtr cavities urn umdn in rockw throiigii \h« Holvpnt action of wntct. Thchc optningH and cavitipH are filled in tlid cour-e of linitt in tlie ""K'" "f ganio way m are gomo (ingureH, by tho depohition of u.uteri.il IVoin f^oln- "'""'''"' ""'" tion. We thus koi niin* rai dt'poHiis or ore 1 odies of variouH Ioihih. If HHorieH of hecis of rockH hiiH lioen folded and tlien posod to eroH- ive or Inoaking down aciion. tiirou;,'b tho nj-ency of water and tho atnioh- 'Jiofi'^HBl.! plierr, tho tops of the folds may U^ worn oil. We tlicn get a Htru<;turo siieh K'i'l!"H't'°u. as ia >.hown by the gnoisa in tho following Hgure. ^^-5LI_ ^^ A»Lmie»fone D-Cranit'C C» CSnes. Q*> Ctronit« Qutirry, St'ctiiiii atriiHB lianicliflil (-'niniiioii, Kiii(/itcin. A l.iiiM'St.ru' (nn ii.|iii!Oii-i nii'k.) li-Craniti' (all lifiiuoim rofl, i. C-i^ii,!,., i,i inntiimnrplili- nn'k.l Recks of four afjes are shown in the section, viz : "ikmss ; ^-raiiite, which cuts Ihrouah tho <{neisK and is therefore tho younjjer of the two ; limeBtone which overli(!8 botii the jjranitt^ and the <,'nii.« and contaiiiH fnignients of the former, and is therefore tho youns^est of the three ; hould(>r3 and other loose mafcerial, which overlio tho limestone and are lienco the youngest rocks present. Several common {geological terms may be PKplainid by niear s of the section, e.g. "contact," the point of Unction of tho limestone with the gneiss or granite ; •' unconformity," the beds of limestone l.vine on the up- ,a-''^e'I'ioS"a'l turned edges of tho gneis.s. I'he layers of the gneiss make an angle with tho **■"""• horizontal. The gneiss is therefore said to "dip "at a cert un jing'e. Tlie horizontal direction or the course of the upturned edges, wliidi is perpen- dicular to the line of dip, is spoken of as the "strike." The granite appears at tho surface of tho ground, or forn)S an "ou'crop." A mass of igneous rock exposed at tho Hurfaco in a more or less rounded f.r irregular foim is spoken of as a " boss." Narrow fissures in the gneiss are filled by gran te. Such structures are known as "dikes." The. IbHings in tho gncLss hhnw "anticlines" or ridge-like forms, and "synclines" or trough-like forms. The bedded structure in the limestone is .spoketi of as " strutitication," each bed being called a .stratum. Tho gnei.ss shows a layer like structure also, out the ittyera are not so regular as those in tho limestone, and are more or less finely beat and crumpled. This structure in gneiss and other crystalline rocks is spoken of as ''schistosity "— -the rocks are sail to ^ i have a schistose structure. The granite is not arranged in layers called a " massive " rock. ' id ia IGNK0D8 H0UK8. The Igneous rocks may be divided as we have seen into three groups—- plnton.c, dike and volcanic. The members of each group are again sub- S'T«toul'™ '^^^' according to the percentage of silica they contain, into acid, inter- rooka. i^Jediate and basic rooks. If a rock contains over 65 per cent, of silica, it is spoken of as an acid rock ; if its percentage of silica is between 50 and 65 It IS said to be intermediate in composition, Rocks containing less than 50 per cent, of silica are said to be basic. The names given to Igneous rocks, unless they are perfect glasses, depend on the minerals they contain— hence also on their percentage of silica-and on their structure, that is on the form and arrangement of their constituent minerals. The following tabular arrangement of the Igneous rocks shows the way m which they are classified according to the description given above Only the more commonly occurring rocks are shown in the table, and since manv of the dike rocks have been but little studied, and are moreover difficult to determine without recourse to refined methods of investigation, their position IS not shown in the table. The pink and light colored varieties of these may be simply called granite or syenite dike rocks, while the name " trap " ii.Hy be applied in the field to the fine grained basic rCcks, whoa;3 true character cannot be made ou in hnnd epecimens. Chief fulsi)ar=OiiTH()(i.A.>sK Kflciiar I LACildCr.ASK o u £_ witli \isuaUy Mica (..dom.nate in this rock, and mica when present is usually light colored. The rock .s o ten a storehouse for rare and valuable minerals, among which may be mentioned tin-stone, tourmaline, corundum and beryl. Graphic graMte ,s a variety in which .he gray quartz is so arranged through the white felspar a, to present the app.arance of characters in the ancient Grecian or Phoenician alphabet 188. Sjenite. Coarse grained ; co'or usually reddish or gray. This rock has much the appearance of granite, and differs from it only in the ^sence of quartz Hence a syenite may be called a quartzless granite. Tv t rl riT"^' '^ «''•'=» ^'^ g--^"'^". -- -count of the presence of quartz. .8 high, 65 to 80. making an acid rock, the silica in syenite is in a consider! ably lower percentage. Hence syenite is said to be a rock of intermediate composition-its percentage of silica lying between that of granite and the basic rocks, or those low in silica. A highly interesting rock .nown as nephohne syenite is found in the northern part of the county of Hastings Ontario. In ,t the rare mineral nepheline plays the part of a felspar. 189. Diorite. Usually a rather coarse grained rock and darker in CO or han syenite from which it differs by having plagioclase instead of o thoclase as its felspatbic constituent. Typical diorite consists essentially of I)lagiocla8e and hornblende. ^ 190. Gabbro. Often very coarse grained ; usually dark in color. It contains a lower percentage of silica than diorite. and typical specimens are composed essentially of basic plagioclase and the variety of pyroxene known as diaUage. Where hypersthene is present as an essential con- stituent the rock IS known as norile. Anorthosite, a rock related to gabbro. consists essentially of lime-soda felspar. It may be mistaken for crystalline limestone but ,s harder. Gabbro often contains much magnetite, and it is believed by some authorities that certain magnetite deposits found associated with this rock are of igneous origin, and have been formed at the same time and out of the same molten mass as the gabbro with which they are associated. A Sim, ar theory has been proposed to account for the origin of the nickeliferoua pyrrhotite deposits of Sudbury, which are associated with gabbro like rocks 191. Obsidian. This is a natural gkss. It i., in some cases, a volcanic representative of the plutonic granite, as the two rocks agree in chemical com- posi ion. Their structural difierence, one being a glass and the other a coarsely crystalline granular rock, is to be accounted for by their difference in origin Granite originated deep down beneath the surface of the earth by the gradual or slow cooling of molten matter. On the other hand, the molten material from M hich obsidian was formed poured out at the surface of the earth and lost heat 80 quickly that there was not time for the molecules of the minerals to arrange hemselves so as to form grains. The whole mass became solid in a compara- tively short time. Rhyolite is like granite in chemical con position, but is »cre or less elassv. Somet;m«>" '*■ - --.r^ a - , . . "«« w •• ' ---"—- II...- ... .., vom^^uscd o£ crystals of orthoclaee and granules of quartz set in a giassy groundmass made up of the same ored felspar however 3ualiy light colored, nerals, amon^ which ind beryl. Graphic rranged through the ters in the ancient iish or gray. This Tom it only in the quartzless granite. i presence of quartz, ite is in a consider- ock of intermediate o' granite and the ng rock known as )un(y of Hastings, t of a felspar, ock and darker in gioclase instead of nsiats essentially of ly dark in color, typical specimens ifiety of pyroxene an essential con- related to gabbro, :en for crystalline ignetite, and it is 8 found associated at the same time hey are associated. f the nickeliferoua bbro like rocks, e cases, a volcanic e in chemical com- le other a coarsely flerence in origin. 1 by the gradual or ten material from arth and lost hfat nerals to arrange lid in a compara- •n position, but is ais of ortiiociase 9 192. Pumice. This rock is a porous or vesicular obsidian. Pitch- iLln " ''"T! .*' ^PP«^'*°««' -^-y be looked on as a devitrified Obsidian, name fels.te is sometimes given to a devitriBed glassy rock tne gra.... ..d compact in structure, and consisting of orthoclase intimately' mixed w..,.. .ome quartz. It has a flint-like fracture, and sometimes is very dull or stony m appearance. The term felsite is however, like the names of arTck nari: tr ''"'''''"' ""''' '' '''^'^" ^^^'^^ ''^' ''^ — ^- - oh.J^^: '^'^'^l^''- ^ ^°l«*°i« 'ook which corresponds to syenite in chemical composition, light gray in color and presenting a dull appearance. Sometimes looks somewhat like a fine-grained limestone. 1 9 i. Andesite. This is the volcanic representative of diorite. to .if' ^T^*" ^°"'"«P°'^^« i° chemical and mineralogical composition to gabbro, and is one of its volcanic representatives. It is a dark, heavv close.„rai„ed rock, and is often known under the name of trap, it often which are filled with agates, zeolites or other minerals. Basalt is a character- istic rock on the north shore of lake Superior. 19G. Columnar Trap. diffJ//' T^T ™''^*'^°*^«''^°''^*"i'= --epresentative of gabbro. It minerals""; " " '*""'""' ''^^'^^"^ '' ^°"^'^*^ °^ '^^ *- --"tial roowL t P'^^'^'^'T *"'^ '"^'*'' ^"* °''^'"'^ ™^y ^'^^ b« P-«-t, when the ock IS known as olivine diabase. Diab.se tend, to weather at th surface o 'ons orTl "*" 7 T'''^^ " '^"-"'^ '"^^^^«- ^^-^^ --"-'^ - thin sec- r H r " ' «»croscope the plagioclase is seen to be in lath-like hanSsre i '".f T '^' "'^"" """^ ^ ""'^'^'^'^ «-^-« «f the rock, in hand spe imens, the plagioclase laths may be seen as very fine short white li^es surtr "• " , ' "'^l' "" ''''^ ""'' '' distinguished. Of course if the ^urf ce examined is much ru.sted or decomp^.d the line, do not come out. il d ZenT"? Tn '"'' '" '°"'' " '''' ™^'^' '' ^^-^ ^"^- -d masses indifferent parts of Ontario, notably in the vicinity of Sudbury. A (i U B O U .s R IJ c K H . 198. Conglomerate. This is composed of rounded fragments of various « , . rocks or minerals cementpH tn^ctu^, i i • s "'•' oi v*' 'ous (Samples of uiiuerais cemented together by calcium carbonate, iron oxide or Aqueoun 1 ZlZ '^ "''' °^ '' """^ ^' '^^^'^ •* ^'^■•^'fi^d gravel bed. '"'''■ lya-.OO. Sandstone. Composeil typically of quartz grains of various c ors cemented toge*e.-, but the rock may be more or less^mpure from presence of o her minerals. It possesses a bedded or stratified s'truoture. .rain«^ A '"""^ '" "°'"''°'''^ ^^P'^^^^ °^ °'^y- ^^ is very fine- grained and occurs m very thin layers. 202. Clay. The character and uses of this material are well known. -03. Kaolin. Ordiuury ciay is an impure form of this substance. 204 Liniestone. Rocks of this class differ much in color, grain and composition. Typically they are composed of the minerU calcit .'together wah more or less dolomite. They are formed through the accumulatbrof 10 »^cl-s and other calcareous structures of various fresh ^vater and marine organisms, such as raollusca and corals. Many limestones contain fossils, thus showing their organic origin, but usually the calcareous material is more or less crushed or broken up. Some limeitones have originated entirely through the deposition of calcium carbonate from aqueous solutions. 205. Lithographic Stone (Limestone). 206. Hydraulic Limestone. 207. Doloinitic Limestone. 208. CakaieousTufa. In this rock the "calcium carbonate of which It 18 composed has been deposited from solution. The rock is more or less porous in appeorance. :'09. Shell Marl. This rock is made up almost entirely of the cal- careous sht'lKs of small organisms. 210. Chalk. This has a similar origin to shell marl but is usually purer and more compact. - 21L Tripolite. Ls formed by the accumulation of the shells of minute or^anisn s kru.wn as diatoms. It is composed of silica and is used as a material for polisiiinsr. M KT A M (p K I'll I (• HOCKS, KTC. Amorphic , , .^'f."-^^- ^°"''- ^« ^"»"*'- '^ "'ineralogical composition to granite, rochs. but IS distinguished from this massive rock by having its minerals arranged m a more or less layer-like form. _ 214 Mica SchiHt. Con.poHed essentially of the minerals quartz .and mica. It splits readily into thin layers or foliie. 215. Hornblende Schist. Has a more massive appearance than mica schist, apd Its chief mineral is hornblende. 210. Quartzite. Is a hardened or metamorphosed sandstone. 217. Quartzite (flexible) or Itacolumite. 218. Slate or ArgiDite. The term slate is used somewhat loosely, but should be restricted to a rock which is a metamorphosed shale (clay) It splits or cleaves in directions independent of the original bedding. 219. Crystalline Limestone. This is similar in con.position to ordinary limestone, but is crystalline in appearance. Some varieties used in the arts are known as marble. 220. Oak-schist. Is a variety of crystalline limeafone. con^'f■lM^'^'"^^°"""■^ ^°'^- '^^'^ ^P^'''"'^'^ '« *«ken from the contact ot the vein and country rock at the Deloro gold mine, Marmora Out or J''^'f J''/" '^' '""" °^ ^'''^- ^'"'^ 'P^^''"^'^ '^°^' « ^»>in section or slice of d.aUise pr, pared for examination under the microscope. 223. Photograph. Shown the appearance which a thir. section of diabase presents under the microscope Photographs taken of objects through the microEccpe are known as photomicrographs.