ROCK. 



( See Veins, Mineral and Metallic. ) Thefe vertical feams of 

 rock are fometimes of vail extent, and vary in width from a 

 few inches to fcveral hundred feet. The moil obvious in 

 ference is, that the folid covering of the globe has been rent, 

 and the mineral matter ejected in a ftate of fufion into the 

 interfaces. The fimilarity of the mineral matter in the 

 greater part of thefe fiffures, provincially called dykes, to 

 that of volcanic eruptions, adds much probability to this 

 opinion. Where thefe dykes occur, the original pofition of 

 the rocks on each fide is generally changed. 



The difiocation of rocks and itrata has been moft noticed 

 in mining diftricts, where it is not unfrequently obferved, 

 that a ftratum which has extended with a regular inclination 

 over a large track of country, is fuddenly thrown down feveral 

 hundred yards or more, and often funk below the power of 

 the miner to regain it. Sometimes the ftrata are thrown 

 upwards, and a feries of rocks, which exift on one fide of 

 the fracture, are entirely removed on the other, and have 

 totally difappeared. 



The change in the original pofition of all rocks and 

 ftrata, with refpedt to the level of the fea, is a moft im- 

 portant and incontrovertible fact. Whether the fea has 

 diminifhed or retired into cavities in the earth, or whether 

 the rocks and mountains have been raifed by an expanfive 

 force, which has broken the folid covering of our planet, 

 and lifted them from the watery abyfs to the prefent eleva- 

 tion, are queftions that may long divide the opinions of geo- 

 logiits ; but that the fummits of our higheit mountains 

 were buried for ages under the ocean cannot admit of doubt. 

 On fome of thefe fummits, remains of marine animals are at 

 prefent widely fpread ; indeed, many of the calcareous moun- 

 tains of vail extent and height appear to be almoft entirely 

 formed of marine (hells, which are not heaped together in 

 confufion, but the upper and lower beds of rock contain 

 remains of fpecies diftinct from each other ; this proves that 

 they were not depofited in their prefent fituation by any 

 fudden inundation, but have been collected in fucceflion at 

 diltant periods of time. 



In fchiftofe mountains, confidered as primary, the beds 

 of rock have generally a vertical pofition. Some geologiits 

 fuppofe that all thefe beds were originally depofited hori- 

 zontally, and have been fubfequently elevated by fome 

 mighty convulfion of nature. Saufiure, who has invef- 

 tigated with fo much labour the ftrufture of the Alps, has 

 drawn this conclufion with refpeft to the rocks which com- 

 pofe Mont Blanc, and the mountains in its vicinity ; but 

 we are inclined to believe that the itructure of thefe moun- 

 tains, as defcribed by this intelligent philofopher, may in 

 many inflances be explained in a more fatisfactory manner, 

 by confidering it as the refult of cryftallization on a large 

 fcale, and the vertical beds of rock as plates of enormous 

 cryftals : indeed, Saufiure himfelf appears to have been very 

 frequently impreffed with furpnfe at the tendency to 

 cryftalline arrangement obfervable in thefe mountain mafles, 

 which he defcribes as prefenting regular pyramidal or 

 rhomboidal forms. The cryftalline nature of the rocks 

 themfelves, and the minerals which fill the vertical feams, 

 point to cryftallization as the great agent in the formation 

 of thefe mountains, whether it took place in an aqueous 

 folution, or when the whole was in a ltate of igneous fulion 

 deep under the furface. In one fituation near the great 

 pyramids of granite, on the fouth-eaft fide of the valley 

 of Chamouni, he defcribes a chain of mountains feven or 

 eight leagues in extent, in which the vertical fection of the 

 beds prefents an arrangement exactly fimilar to that of the 

 tticks of an opened fan. The loweft beds are nearly hori- 



zontal, but they gradually rife till they become vertical at 

 the fummit. He fays, that many other inflances might be 

 cited of a fimilar arrangement. Now, we can fcarcely con- 

 ceive it pofiible for any overturning of the mountain to have 

 produced fuch a pofition of the beds ; but in the cryftalliza- 

 tion of minerals on a fmaller fcale the diverging itructure 

 is common. There is, however, one fact, which, if it can 

 be fatisfactorily afcertained, will prove the truth of Sauf- 

 fure's opinion, that the vertiaal pofition of fome of thefe 

 rocks was not their original one. In his defcription of the 

 rocks in Valorfine, he fays, that he found vaft beds of pud- 

 ding-Hone in a vertical pofition, between fchiftofe rocks in 

 a fimilar pofition ; the beds on one fide are gneifs or mica- 

 flate, and on the other flate and fand-ftone. The bed of 

 pudding-ftone is ioo toifes thick ; it confifts of a fine pafle 

 or cement, compofed of extremely fine fchiftofe fand-ftone, 

 with minute plates of mica, that are perfectly parallel to 

 the feams which divide the beds. 



The fragments vary in fize, from that of a grain of fand, 

 to fix or feven inches in diameter ; fome are angular and others 

 rounded, refembling the boulders on the fhores of the lake 

 of Geneva. Thefe fragments and boulders are of gneifs, 

 mica-flate, and quartz, but none are of clay-flate. Saufiure 

 has obferved, that it is impoffible for thefe rounded boulders 

 to have been originally placed in a vertical fituation. If 

 they were formed mechanically by the action of water, like 

 thofe on the borders of the lake of Geneva, this inference 

 is undeniable ; and we mull farther admit with Saufiure, that 

 the niafs of this mountain, which is 1181 toifes above the 

 level of the fea, has been overturned by the fame revolution 

 which has given a vertical direction to the whole ; for all thefe 

 beds having the fame inclination and direction as that of 

 the pudding-ftone, we are compelled to grant that their 

 original fituation was the fame, and that they have fuf- 

 fered the fame change from the fame caufe. The only 

 objection to this inference that can, with any probability, 

 be urged, is, that thefe rounded ftones may not really be 

 water-worn boulders and pebbles, but were formed in the 

 fame manner as the bolls of bafalt and granite, which are 

 fometimes imbedded in bafaltic or granitic rocks, and which 

 are acknowledged to be the refult of a tendency to cryf- 

 talline arrrangement. 



Saufiure, however, was fully convinced that they were 

 real boulders, and his guide, on feeing a number of the fame 

 rounded fragments on the fummit of a mountain in the 

 vicinity, exprefted his furprife at finding, in that elevated 

 fituation, the pebbles and boulders he was accuflomed 

 to fee on the borders of the lake. The boulders, in this 

 fituation, were evidently the remains of fimilar beds of pud- 

 ding-ftone, of which the pafte or cement had been decom- 

 pofed and wafhed away. 



It is obferved, that fecondary ftrata always rife towards 

 the primitive mountains in their vicinity, which adds proba- 

 bility to the opinion, that the latter were forced up from 

 great depths, and have raifed with them the rocks by which 

 they were originally covered, and which now border the 

 primary. The difciples of Werner, however, contend, that 

 the elevation of fecondary ftrata is occafioned by the incli- 

 nation of the rocks on which they were originally depofited, 

 and that they have undergone no fubfequent change. 



Succejfion of Rocks. — There are certain rocks, which com- 

 monly occupy the loweft relative pofition in various parts 

 of the world ; the rocks which cover them are frequently 

 arranged in a fequence, which has much fimilarity in diftant 

 diftricts. 



Werner, who formed his fyftem from obfervations made 



in 



