ROCK. 



clufion, that rocks of porphyry, fienite, trap, bafalt, and 

 lava, had all a fimilar mode of formation, as they agree in fo 

 many characters of compoiition and ftrudture. See Volcano, 

 Volcanic Produils, Trap, and Sienite. 



As Mont Blanc in Switzerland is the higheft mountain 

 on the continent of Europe or Afia that has been examined 

 by any eminent mineralogiil, we (hall give a Ihort account of 

 its ftru&ure, and the rocks which compofe its fummit, as de- 

 fcribcd by Sauflure, who afcended it in 1786, and who had 

 devoted many years to the lludy of the rocks in its vicinity. 

 Thefe rocks, fituated nearly 2400 fathoms above the fea, 

 are interefting, as being the molt elevated that have been 

 obferved by naturalilts ; and the fcarcity of rock fpecimens, 

 brought from great elevations, renders his account the more 

 deferving of notice. A more numerous collection of fuch 

 fpecimens from various parts of the globe would throw much 

 light on this department of fcience. 



After having gained the higheft point of the mountain, 

 " the firft thing which {truck me," fays Sauflure, " in 

 the entire view of the high mountainous fummit s which 1 

 had under my eyes from the fummit of Mont Blanc, (the 

 higheft of them all, ) was the kind of diforder which pre- 

 vailed in their difpofition. When we contemplate the range, 

 of which Mont Blanc forms a part, from lefs confiderable 

 elevations, it appears as if thefe colofTal mountains were htu- 

 ated in a line, and formed a chain ; but this appearance va- 

 nifhes entirely from the bird's-eye view which is here pre- 

 fented. The mountains to the north of Mont Blanc, in 

 Savoy and Switzerland, are indeed united among themfelves, 

 fo as to form mountain chains, but the primitive mountains 

 do not prefent this appearance. They are diftributed in 

 great mafles, or in groups of various itrange forms, de- 

 tached from each other, which appear at lealt but acciden- 

 tally united, without any regularity. Thus on the eaft, 

 the lofty peaks called les Aiguilles de Chamouni, the moun- 

 tains of Argentiere, of Courtes, and of Taleul, form one 

 triangular group, almoft detached from Mont Blanc, and 

 only connected with it at the bafe by a narrow ridge. 



" On the fouth-ealt likewife, mount Zuc, La Rogne, and 

 the other primitive mountains to the north of the fummit of 

 I'Allee Blanche, form a group nearly triangular, feporated 

 from Mont Blanc by the valley of the glacier of Miage, 

 and which is only connected with Mont Blanc by the bafe 

 of the mountains which clofe that glacier to the north. 



" Mont Blanc itfelf forms a mafs almoft ifolated, the dif- 

 ferent parts of which are not in the fame line, and have no 

 relation of fituation with the other groups. 



" On calting my eyes ftill further, I confirmed the fame 

 obfervation. The primitive mountains of Switzerland and 

 Italy, which I had fufficiently near to be under my eyes, 

 prefented only feparated mafles, or detached groups, wiih- 

 eut order or regularity. Notwithftanding this irregularity 

 in the forms and distributions of the grand mafles, I ob- 

 ferved certain important refemblances in the ftructure of 

 their parts. All that I diftinftly faw, appeared compofed 

 of plates (ftuilkls), arranged in the fame manner nearly 

 from north-eaft to fouth-weft. I had particular plealure 

 in obferving the fame ftructure in the Aiguille du Midi, 

 which I had formerly endeavoured but in vain to ftudy, 

 being prevented by the inacceflible walls of granite that 

 furround the bafe. I had a view of the Aiguille du Midi 

 on the fecond day of my afcent, and never loft fight of it as 

 I proceeded. I aflured myfelf that it is entirely compofed 

 of magnificent plates of granite, perpendicular to the ho- 

 rizon, and directed from north-ealt to fouth-weft. Three 

 of thefe plates, feparated from each other, form the fummit, 



decreafing gradually in height from the fouthern face, on 

 the fide of the Col du Geant. 



" When feen from the bafe, thefe plate9 had the appear- 

 ance of being bent, like the leaves of an artichoke ; but this 

 mult be an optical illufion, for all thofe which I could now 

 fee diltinftly appeared ftraight ; and if there were any ex- 

 ceptions, they were only local, and of fmall extent. 



" This great phenomenon of the vertical pofition of the 

 plates can only be explained, by admitting a great over- 

 turning of the whole mafs, which has lifted them from their 

 original horizontal pofition. 



" Another queltion, which I wanted anxioufly to re- 

 folve, was alfo now anfwered. Thefe great plates of 

 rock preferved the fame nature and quality at the fummit 

 as at the bafe, where I had fo frequently examined them. 

 This obfervation proves a remarkable property in moun- 

 tains with vertical beds : each bed preferves the fame nature 

 from the bottom to the top. 



" From this magnificent obfervatory I could comprife in 

 one view the whole of that great phenomenon — which I had 

 before contemplated but in parts — the elevation of the beds 

 forming the mountains on the fide of Mont Blanc. On 

 whichever part I turned my eyes, I faw the fecondary 

 chains of mountains, and even the primitive of the fecond 

 order, railing their beds againft Mont Blanc, and the high 

 himmits in its vicinity. Such were the mountains on the 

 north of Repefoir, of Pally, of Servoz, and Le Buet ; on 

 the fouth, the Col du Ferret, Great St. Bernard, and thofe 

 of the chain of Cremont, more remote, and beyond the 

 mountainous chains, whofe efcarpments turn to Mont 

 Blanc. We faw others, whofe efcarpments were turned in 

 a contrary direction. Thefe appearances are in perfedt ac- 

 cordance with the theory which fuppofes that the cruft of 

 the globe has been broken, and the beds of rock raifed from 

 their horizontal pofition. It follows from hence, that the 

 horizontal diftance from the bottom of the valley of Cha- 

 mouni (if that were once the ancient furfacc of the globe) 

 fhould have fome correfpondence with the height of Mont 

 Blanc ; and that this diftance is nearly the thicknefs of the 

 ftony cruft, which has been broken and elevated ; and that, 

 coniequently, Mont Blanc, which actually rifes about a 

 league above the furface, was in its original pofition buried 

 two leagues beneath it." 



The naked rocks on the fummit, which form two kinds 

 of arretes or crefts of a dark colour, are of granite. The 

 felfpar in this granite is white, inclining to grey, green, or 

 reddilh : when expefed to the flame of the blowpipe, it yields 

 a colourlefs tranfparent glals, but full of bubbles. The 

 felfpar is fometimes intermixed with a kind of earthy fteatite. 

 The quartz in this granite is femi-tranfparent and whitilh, 

 and appears rather unttuous in the frafture. Very minute 

 fragments were rounded by the flame of the blow-pipe : it 

 is, therefore, more fufible than rock-cryftal. Thefe gra- 

 nites are alio intermixed with green and black hornblende, 

 and with chlorite, which feems to fupply the place of mica, 

 as the latter fcarcely appears, and only in minute fpangles. 

 In fome places, thefe granites graduate into irregularly 

 fchiftofe rocks, compofed of quartz and felfpar, whofe 

 feams are filled with a brown argillaceous and ferruginous 

 earth, that melts into a black glafs. The granite on the 

 aftual fummit is compofed, like the above, of quartz, fel- 

 fpar, and hornblende or fteatite. Felfpar conftitutes about 

 three-fourths of the mafs : the hornblende and fteatite form 

 too fmall a portion to be eftimated, the quartz forming 

 nearly the whole of the remaining fourth part. On the 

 northern fummit, befides the above fpecies of granite, he 



met 



