FUNDAMENTAL FORMS OF ROCKS. 



7S 



earth, but of the atmosphere, surcharged with 

 moisture and of much greater extent than it is 

 at the present day. If at the present time, on 

 surfaces as extensive as Europe, we scarcely 

 find four openings (volcanoes) through which 

 eruptions of fire and molten matters can take 

 place, the firm crust of the earth was traversed 

 in former periods by vast open fissures, through 

 which mountain chains were upheaved, or into 

 which streams of molten rock — granite, por* 

 phyry, basalt, and melaphyre — were injected, 

 and by which they were variously stopped and 

 filled up. At former epochs, in the much and 

 variously fissured, thinner, and upwardly and 

 downwardly fluctuating crust of the earth, there 

 were almost everywhere passages of commu- 

 nication between the molten interior and the 

 atmosphere. Gaseous emanations arising from 

 very dissimilar depths, and therefore bringing 

 chemically different substances, then animated 

 the Plutonic formative and transformative pro- 

 cesses. The sedimentary formations, too, the 

 precipitations from liquids, which we designate 

 travertin, and which we see proceeding in the 

 neighbourhood of Rome as well as of Hobart 

 Town in Australia, from cold and hot springs 

 and river waters, give but a very poor idea of 

 the origination of the floetz formations. Our 

 seas, in virtue of processes which have not yet 

 been examined generally enough, or with suffi- 

 cient care, gradually form by precipitation, by 

 overflowing and by cementation, small calca- 

 reous banks, which, at some points, almost ap- 

 proach Carrara marble in hardness(2^3). This 

 process goes on upon the Sicilian coasts, the 

 Island of Ascension, and King George's Sound 

 in Australia. On the coasts of some of the 

 "West India islands these formations of the 

 present ocean now enclose earthenware ves- 

 sels and other products of human manufactu- 

 ring industry ; and in the Island of Guadaloupe, 

 even skeletons of the Carib race of men. The 

 negroes of the French colonies characterize 

 this formation as the " Masonry of God" (Ma- 

 <;onne-bon-Dieu) (^i*). In the Island of Lan- 

 cerote, one of the Canaries, there is a small 

 oolitic stratum, admitted to be a product of the 

 sea and of storms, but which, despite its new- 

 ness, reminds us of the Jurassic limestone(2^5) 



The compound rocks are determinate asso- 

 ciations of certain simple minerals — felspar, 

 mica, solid silicic acid, augite, and nepheline. 

 Very similar rocks, i. e. rocks made up of the 

 same elements but otherwise grouped, are pro- 

 duced by volcanic processes under our eyes, at 

 the present time, just as they were in former 

 epochs of the world's history. The independ- 

 ence of rocks in respect of geographical posi- 

 tion or relationship, is so great, that, as we 

 have already observed("^), the geologist sees 

 with amazement, to the north and south of the 

 equator, in the farthest zones of the earth, the 

 same familiar appearances in the rocks, the 

 repetition of the minutest details in the pe- 

 riodic series of the Silurian strata, and in the 

 effects of contact with augitic masses, the prod- 

 ucts of eruptions. 



If we now take a closer view of the four fun- 

 damental forms of rock (the four phases in the 

 formative process) in which the stratified and un- 

 stratified portions of the crust of the earth pre- 

 sent themselves to us, we may designate among 



the endogenous or eruptive rocks, (the massive 

 and abnormal rocks of some modern geologists), 

 the following principal groups, as immediate 

 evidences of subterraneous activity, viz. : 



Granite and Syenite — of very different rel- 

 ative ages, but frequently penetrating both gran- 

 ite and syenite of more recent formation in 

 veins(2i7). Along with these it is also proper 

 to consider the forcing or upheaving power. 

 "Where granite protrudes in evenly vaulted 

 ellipsoids, in great masses, like islands, wheth- 

 er this be in the Harzforest, or in Mysore, or 

 in Lower Peru, it is always covered with layers 

 that have become fissured into blocks. Sur-h 

 a rocky sea probably owes its origin to a con- 

 traction of the upper surface of the granitic 

 vault, which, on its protrusion, and originally, 

 must have been very much expanded"(2i''). 

 In Northern Asia also(=^^^), in the charming, 

 the romantic neighbourhood of Lake Kolyvan, 

 on the north-western declivity of the Altai 

 range, as also on the slopes of the maritime 

 chain of Caraccas, near Las Trincheras(=^=">), I 

 observed the granite subdivided into blocks or 

 pilesj in consequence, possibly, of such con- 

 tractions, but which in these cases appear to 

 have extended deeply into the interior. Far- 

 ther to the south of Lake Kolyvan, towards the 

 confines of the Chinese province Hi, between 

 Buchtarminsk and the river Narym, the char- 

 acters of the entire mass of ejected rock, which 

 is here unaccompanied by gneiss, are more stri- 

 king than I have observed them in any other 

 part of the globe. The granite, always scaling 

 and crumbling on the surface, and splitting up 

 into tabular masses, rises in the steppes here 

 in low semi-globular hillocks, not more than six 

 or eight feet high, there in basalt-like knolls, 

 which run out at opposite sides, as it were, 

 into thin wall-like effusions(2"). By the cat- 

 aracts of the Orinoco, as well as in the Fichtel- 

 gebirge (Seissen), in Gallicia, and betwixt the 

 Southern Ocean and the lofty platforms of Mex- 

 ico (at Papagallo), I have seen granite in great 

 depressed globular masses, which, like basalt, 

 split or scaled off in concentric layers. In the 

 valley of the Irtisch, between Buchtarminsk 

 and Ustkamenogorsk, the granite covers the 

 clay-slate for a mile in length("2)^ and pene- 

 trates the same strata from above in slender 

 veins, which are numerously branched, and 

 wedge-shaped at their extremities. I have ad- 

 duced these particulars by way of examples, 

 only that I may illustrate the individual char- 

 acters of an eruptive rock in one of the most 

 widely diffused of the mineral masses. In the 

 same way as the granite overlies the schists in 

 Siberia, and in the Department of Finisterre 

 (Isle de Michau), so does it cover the Jurassic 

 limestone in the mountains of Oisons (Fer- 

 ments), and syenite, and chalk with syenite in- 

 terposed, near Weinbohla, in Saxony("^). In 

 the Ural mountains near Mursinsk, the granite 

 shows drusy cavities, and the druses here, like 

 the fissures and druses of newer volcanic pro- 

 ductions, are the Plutonic seat of numerous 

 beautiful cystals, particularly of beryl and topaz. 



QuARTzosE Porphyry, from its relations of 

 stratification, having frequently the character 

 of veins. The base is generally a finely gran- 

 ular mixture of the same elements which pre- 



