151 



u conglomerate, breccia (in part), 



. -. various da\s, 1 1 nidHtones, shale-, i\c. 

 .'ttrtnl rock-, as rock debris, erratics, moraines, 

 boolder da\ , AI-. 



Chemically-formed Hock*. 'I'lie rocks included 

 under this subdivision are sometimes cartliy in 

 ter, liut more freiiuently show a crystalline 

 act >ul> crystalline texture. Among the 



t \piral kinds are kaolin and various other 

 d:i\-, stalactites and stalagmites, calc-tufa and 

 it- varieties, geyserite (siliceous sinter), rock-salt, 

 dolomite, gypsum, flint, chert, various ironstones, 



Organically-derived rocks are made up of the 

 p-lic- of animal and plant life. They include a 



Si. -at \ariet \ of limestones, diatom-earth (tripoli), 

 int (in part), various phosphatic deposits, peat, 

 lignite, coal, anthracite, oil-shale, various iron ores, 



No hard and fast line can be drawn between 

 the older and younger products of epigene action. 

 It U obvious tfiat conglomerate and sandstone are 

 merely compacted gravel and sand ; breccia is only 

 cuiisoiidateil nick-debris; while lignite and coal are 

 .-imply vegetable matter more or less mineralised. 

 The thick fossiliferous limestones of the earth's 

 crust are paralleled by the coral-reefs and organic 

 oozes of existing oceans, and have evidently had a 



J. similar origin. Every derivative rock, indeed, can 

 be compared with a like product of modern epigene 

 action. The older products, it is true, are most 

 frequently solidified, while the younger are oftener 

 more or less incoherent and unconsolidated. But 

 this difference is not essential, and is only what 

 might have been expected. The older products 

 have for a long time been exposed to the action of 

 percolating water. In many cases they have been 

 .-uKjected to the influence of subterranean heat and 

 enormous pressure, and we need not wonder, there- 

 fore, that they should have acquired a more or less 

 indurated character. But solidification does not 

 invariably characterise the older products, nor are 

 modern accumulations always incoherent. There 

 are indurated conglomerates and sandstones of very 

 recent formation, and some modern coral-rock is as 

 hard and compact as the older limestones. Hence 

 the term rock is applied to all the products of 

 epigene and hypogene action alike, whether the 

 material so designated be yielding, as clay and peat 

 and blowing sand, or hard and resisting, as con- 

 glomerate, limestone, or granite. 



( 3 ) MetamorpMc Rocks. All rocks sooner or later 

 undergo some process of alteration whereby their 

 original character becomes modified. Thus, by the 

 chemical action of percolating water some lime- 

 stones have been more or less changed into dolo- 

 mite ; olivine rocks have been altered into serpen- 

 tine ; some sandstones have been converted into 

 quartzites. Derivative rocks at the point of contact 

 with igneous rocks are very frequently altered to a 

 greater or less extent. Thus, ordinary limestone 

 becomes crystalline marble, coal is changed into 

 graphite, sandstone into quartzite, clay and shale 

 into porcellanite. When alteration of a rock, how- 

 ever caused, has proceeded so far as to produce a 

 rearrangement or the constituent elements of a 

 ">ck, and to develop a crystalline or semicrystalline 

 tractate, such extreme alteration is termed nn-tn- 

 iiinr/i/i/x/ii. and the rocks so affected are described 

 as metcanorphic. UOCKS of this kind are sometimes 

 confusedly crystalline or massive in structure, and 

 in hand specimens might le mistaken for piutonic 

 igneous rocks ; but by far the larger number are 

 distinguished by a peculiar flaky or pseudo-lamin- 

 ated structure which is termed Foliation (q.v. ). 

 In foliated or schistose rocks the constituent 

 minerals are arranged in alternate lenticular layers 

 which merge into each other. Such arrangement, 



it miiHt be understood, ban no relation to the layer* 

 of deposition BO frequently present in derivative 

 rocks like shale, sandstone, &c. The foliated 

 si i nctiire has been superinduced in rocks, some of 

 which may have been igneous and other* aqueous 

 in origin. It i obvious, however, that the study 

 of causes now in action can throw little light on 

 tin- origin of foliation. We may study the change* 

 induced in rocks by contact with the iiroductH of 

 modern volcanic action, ami these will doubtless 

 enable us to understand how certain alterations 

 in locks have been brought about; but Hchihtotuty 

 is not superinduced in rocks in the neighbourhood 

 of modern volcanic orifices. In Britain and other 

 countries, however, denudation has ex|MMed the 

 interior and basal portions of ancient volcanoes, 

 and we can now study in detail the fractured and 

 baked rocks through which heated gases, molten 

 matter, &c. have been erupted. Nay, in some 

 cases, we can even examine enormous masses of 

 piutonic crystalline rock which are believed to be 

 the reservoirs from which the molten matter of our 

 ancient volcanoes was pumped to the surface. Such 

 great piutonic masses are frequently surrounded by 

 a zone or belt of crystalline schistose rocks, such as 

 gneiss, mica-schist, &c. The rocks are most .crystal- 

 Fine and schistose in the immediate proximity of 

 the igneous mass, but gradually lose these char- 

 acters as they recede from its neighbourhood, until 

 by-and-by they pass into ordinary derivative rocks 

 such as graywacke, shale, &c. Some schistose 

 rocks, therefore, undoubtedly owe their origin to 

 contact with deep-seated igneous masses. Again, 

 it has been observed that where rocks, whether 

 igneous or derivative, have been subjected to 

 enormous crushing and pressure, they not infre- 

 quently become crystalline and schistose. There 

 are some schistose rocks, however, the origin of 

 which is still very obscure. Geologists cannot yet 

 assert, therefore, that all schistose rocks are meta- 

 morphic (see ARCHAEAN SYSTEM). Among the 

 most characteristic metamorphic rocks are quartzite, 

 marble, phillite, mica-schist, talc-schist, chlorite- 

 schist, hornblende-schist, actinolite-schist, gneiss, 

 granulite, eclogite, &c. 



STRUCTURAL or GEOTECTONIC GEOLOGY is that 

 branch of the science that deals with the arrange- 

 ment or structure of rock-masses. 



Structure of Igneous Rocks. Igneous rocks are 

 grouped under two series viz. (a) Contemporane- 

 ous and (6) Intrusive eruptive rocks. 



(a) Contemporaneous eruptive rocks are either 

 crystalline or fragmental. The crystalline rocks 

 are simply old lava-flows, while the fragmental 

 rocks consist of tuff and its varieties. They are 

 in short tbe products of volcanic action, and have 

 been erupted at the earth's surface, accumulating 

 either upon the land or under water. Many or 

 these rocks have apparently been erupted from 

 vents of the ordinary modern type, but others 

 appear to have come up along lines of fissure in 

 the earth's crust the lavas overflowing the surface 

 in broad floods. Successive outflows of this kind, 

 accompanied frequently by the ejection of frag- 

 mental materials, have built up some great 

 plateaus. Contemporaneous lavas are generally 

 more or less scoriaceous or porous above and below. 



(b) Intrusive eruptive rocks are also crystalline 

 and fragmental. ntdu are approximately cylin- 

 drical funnels filled with either crystalline igneous 

 rock or fragmental materials, or with both. They are 

 obviously the plugged throats of old volcanoes, the 

 upper parts of which have been removed by denuda- 

 tion. Intrusive Sheets are more or less lenticu- 

 lar masses of crystalline igneous rook which have 

 been erupted amongst strata in a direction more or 

 less closely conformable with the planes of bedding. 

 They seldom show any scoriaceous structure, and 



