ROCK. 



claflification was fupported, and the folemn trifling often 

 employed to determine whether certain rocks belonged to 

 the tranfition or flcetz clafl'es." (Bakevvell's Introduction 

 to Geology, 2d edit.) Indeed the term primary, or pri- 

 mitive, is objefted to by fome geologilts, who confider 

 thefe rocks as having been in a ftate of igneous fufion by 

 fubterranean fire, which gave to them their cryftalline ftruc- 

 ture; but the heat afted with lefs force on the rocks 

 by which they are covered, hence the latter are more 

 earthy. Nor are appearances wanting to iupport this 

 opinion. Well-authenticated inftances are known of veins 

 mooting from the lower into the upper recks ; and 

 though much labour has been employed to controvert this 

 faft, it is now undoubted. The granite of Cornwall 

 fends up veins into the fuperincumbent fchiflus, or killas ; 

 hence it was inferred that the latter, which is coniidered as 

 a fecondary rock, was older than the granite or primary 

 rock. It is evident the killas cannot be of more recent 

 formation. Both rocks were probably contemporaneous ; 

 in which cafe, we may conceive that veins might fhoot from 

 the lower into the upper, previoufly to their acquiring a 

 compaft folid ftate. Nor will the abfence of organic re- 

 mains alone prove the prior formation of thefe rocks ; for 

 among rocks abounding in petrifactions of animals or ve- 

 getables, many beds of rock occur in which no fuch remains 

 are ever met with, although we are certain to find them in 

 the ftrata immediately above and below. We have no 

 means of afcertaining that the fimilar rocks of different 

 diftrifts were formed at the fame time ; nor can we be 

 certain that they have not once contained organic re- 

 mains, that were deftroyed during the procefs by which they 

 acquired their prefent cryftalline ftrufture. We may, how- 

 ever, with apparent probability, infer, that their formation 

 was prior to the exiftence of animals and vegetables in our 

 planet, in its prefent ftate ; becaufe the rocks which im- 

 mediately cover them contain almoft exclufively the organic 

 remains of zoophytes, or thofe atiimals which are confidered 

 as forming the firft link in the chain of animated beings. 



It is only from the exiftence of thefe organic remains, 

 we can infer, with any certainty, that the rocks in which 

 they occur were formed in fuccefiion. The difference of 

 thefe remains in the upper and lower rocks mark diftinft 

 epochs in the natural hiftory of the globe. In the loweft 

 rocks, which have a cryftalline granitic ftrufture, no animal 

 or vegetable remains have ever been found ; but the cal- 

 careous rocke, which immediately cover them, contain fome 

 few remains of zoophytes and fhell-fifh ; and in the argil- 

 laceous flate rocks, alfo, we occafionally meet with vege- 

 table matter. In the fand-ftone ftrata over thefe, the re- 

 mains of vegetables are abundant. Above thefe again, in 

 the upper calcareous ftrata, entire fifh are occafionally 

 found, with zoophytes and fhell-fifh, of a fpecies different 

 from thofe in the lower ftrata. In the thick beds of alu- 

 minous (hale and clay over thefe, occur the remains of the 

 fhark and alligator ; and in the alluvial foil which covers the 

 whole, the bones of the elephant, the rhinoceros, the hip- 

 popotamus, and the maftodonton or mammoth, and of va- 

 rious unknown quadrupeds, are found both in our own 

 country, and in many parts of Europe, Afia, and America. 

 But neither in the upper nor lower rocks, nor in alluvial 

 ground, have any remains of human fkeletons been found, 

 except where mines had formerly been worked, or in fitua- 

 tions where their occurrence could be explained by recent 

 caufes ; fuch as inundations and volcanic operations, as on 

 the (hores of Guadaloupe and in the neighbourhood of 

 Vefuvius. 



We believe our own countryman, Mr. W. Smith, was the 



fir ft perfon who obferved that remains of diftinft genera and 

 fpecies of animals were peculiar to certain ftrata, and that the 

 occurrence of the fame remains was fufficient to identify 

 the fame ftratum, throughout a whole diftrift, wherever it 

 could be examined. The fagacious naturalift M. Cuvier 

 has applied the fame difcovery to illuftrate the geologv 

 of the country round Paris, a diftrift molt remarkable 

 for the number and variety of foffile fkeletons, and other 

 reliquix which it contains. See Strata. 



It is not a little remarkable, that few of the reliquiae, whe- 

 ther in the upper or lower rocks, belong to exifting fpecies 

 of marine or terreftrial animals ; and the vegetable petrifae- 

 tions found in northern latitudes refemble meft thofe of tro- 

 pical climates. 



The natural hiftory of rocks comprifes, the fubftances of 

 which they are compofed, their internal and external ftruc- 

 ture, their pofition, and the order of fucceffion, from the 

 loweft rocks with which we are acquainted upwards to the 

 furface — their decompofition, and the mode of their forma- 

 tion. The latter is the province of fpeculative geology, fo: 

 it is only in one clafs of rocks, the volcanic, that we have 

 any experience of their aftual formation. 



The mineral repofitories peculiar to certain rocks will br 

 defcribed under the article Veins, Mineral and Metallic. 



The Competition of Rods. — Rocks are either iimple or 

 compound. Thofe rocks which are compofed of one mii.c 

 ral fubftance, are called fimple, fuch as flate, ferpentine, 

 lime-ftone, &c. although thefe minerals may be compofed of 

 various elementary fubftances. Thus clay-flate, or flate, con- 

 fifts of filex and alumine, combined with oxyd of iron and 

 carbon ; but the combination is fo intimate, as to prefent 

 the appearance of an homogeneous fubftance. When two 

 or more minerals enter into the compofition of rocks, they 

 are called compound. Thus granite is compofed of quartz, 

 felfpar, and mica, clofely united together, but each of thefe 

 minerals preferves its own peculiar character. The ele- 

 mentary fubftances of which all rocks, both fimple and 

 compound, are formed, are, the earths filex, alumine, lime, 

 magnefia, the oxyd of iron, carbon, and fulphur. (See Silex, 

 Alumine, &c.) The newly difcovered earths and me- 

 tallic ores, except iron, nrely form the fubftance of rocks, 

 but are found in the veins and fiflures by which they are in- 

 terfered. 



From the above elementary fubftances, either feparately 

 or combined, allthe fimple minerals are formed which com- 

 pofe rocks and mountains. But it may be remarked, that 

 filex enters moft largely into the compofition of all the lower 

 cryftalline rocks (except granular lime-ftones) : it forms 

 more than two-third parts of the lower cruft of the globe. 



The moft important fimple minerals which form rocks and 

 mountains, are quartz, felfpar, argillaceous fchift, or clay- 

 flate, lime-ltone, hornblende, ferpentine, chlorite, mica, talc, 

 hornllone, jafper, flint, bituminous fhale, and alum-flate. 

 Bafaltic or trap rocks, and lava, are fometimes compofed 

 of one apparently homogeneous fubftance, but more fre- 

 quently they prefent the appearance of compound rocks. 

 For the characters of thefe minerals, fee Quartz, Felspar, 

 &c. 



When two or more of thefe minerals are intermixed toge- 

 ther, they form compound rocks ; in thefe the minerals are 

 either clofely united together, without any vifible cement ; 

 or aggregated and held together by the intervention of an- 

 other mineral fubftance, which ferves as a cement, and is 

 fometimes called the pafte. 



The Structure of Rods, internal and external. The hitter, 

 or the ftrufture of rocks en maffe, is 36 diftinft from the 

 former, as the order of architecture of a building is diftinft 



from 



