ROCK. 



met with a kind of trap rock, compofed of minute cryftals 

 of white felfpar and hornblende. On the fouthern fummit 

 he alfo found rocks of petrofilex (hornltone), of a pearl- 

 grey colour. It is deferving notice, that the definition of 

 thefe granites, as given by Saufl'ure, would, according to 

 the Wernerian fyftem, bring them under the denomination 

 of fecondary granites ; though furely, if any granite can 

 properly be conlidered as primary, it is that of Mont Blanc, 

 and the mountains in its immediate vicinity. 



The following extract from profeflbr Jamefon's Geog- 

 nofy comprifes a {hort view of the order of fucceffion of 

 rocks, as laid down by the German geologifts ; with ob- 

 fervations on the fucceflion of the rocks of Saxony and 

 Hanover, dcicribed in the quaint language peculiar to the 

 fchool of Frcyburg. 



" In the primitive clafs of rocks, we obferve feveral rocks 

 always difpofed in conformable and unbroken ftratification, 

 and in which the newer and newer ftrata have always a lower 

 and lower level. Gneifs, mica-flate, and clay-flate, are of 

 this kind. The granite Itretches under them uninter- 

 ruptedly, and fometimes rifes through them, or juts up in 

 the form of fingle caps or great mafles ; fo that the gneifs 

 and other rock-: arc difpofed on its furface, fometimes in a 

 concave, fometimes in a convex direction, fometimes faddle- 

 fhaped, and frequently mantle- fhaped. It is evident from 

 thefe relations of the ftrata, that granite will frequently 

 form the greateft heights on the furface of the globe. 



" Porphyry has a very different kind of ftratification from 

 the preceding rocks. It occurs iometimes broken, fome- 

 times unbroken. When broken, it prefents caps, upfillings, 

 and fhield-fhaped ftratification. When unbroken", it forms 

 widely extended mafles : its pofition is unconformable, and 

 overlying. 



" Grey-wacke occurs fometimes in a conformable, fome- 

 times in an unconformable pofition ; alfo in caps, upfillings, 

 and fhield-fhaped, and frequently mantle-fhaped, ftrata, fur- 

 rounding the older mountains. 



" The lime-ftone and fand-ftone formations are ufually 

 difpofed in a mantle-fhape, round the older formation ; 

 fometimes they are broken, but more frequently unbroken. 

 They are very common and widely diflributed formations. 



" Coal, again, (hews a very peculiar character : its ori- 

 ginal extent is not confiderable ; it even appears interrupted, 

 or broken ; but its internal characters fhew that its prefent 

 apparently broken appearance is its original one. It occurs 

 commonly in trough and bafin-fhaped hollows, and its ftrata 

 have confequi nth a concave direction. 



" The rocks of the newelt floetz-trap formation arc diflin- 

 guifhed from the older by their unconformable, overlying, 

 and broken ftratification. In thefe refpecls they nearly 

 agree with porphyry. When the continuity of the forma- 

 tion is broken, it occurs in caps, upfillings, and rarely Ihield- 

 fhaped. 



" The defcription we have now given of the fucceflion and 

 ftrufture of the different clafles of rocks, will enable us to 

 inveftigate the ilrutturc of the whole mountain groups. 

 W< fhall illuftrate this by a very fliort defcription of two 

 well-known tracks of country, the Hart/, and the Saxon 

 - Lirge, or metalliferous mountains. 



" Defcription of the Hartz. — An immenfe mafs of granite 

 fornix the centre of this country ; it rifes through the Other 

 ftrata, ami i. elevated a confiderable height above them 

 all, iurming the famous mountain — the Brocken. Mantle- 

 fhaped ftrata of clay-flate arc wrapped around this central 

 mafs. It is worthy of remark, that gneifs and mica-flate, 

 two of the moft confiderable of the older formations, arc 



wanting in this country. To the clay-flate fucceeds tranfiti'on 

 lime-ftone, then grey-wacke, and grey-wacke flate ; and the 

 whole of thefe are wrapped around the granite in mantle- 

 fhaped ftrata, and invariably with lower and lower outgoings, 

 correfponding to the newer and newer ftrata. The flcetz 

 rocks that immediately fucceed the tranfition furround them 

 in mantle-fhaped ftrata. Immediately on the neweft of the 

 tranfition rocks refts the oldeft of the flcetz, the old red 

 fand-ftone ; to this fucceeds the other flcetz formation, in the 



following order, according to their relative age: firft flcetz 



lime-ftone, firft floetz gypfum ; fecond or variegated 

 fand-ftone, fecond or newer floetz gypfum, fecond floetz 

 lime-ftone. Thefe floetz rocks are the links that conned the 

 tranfition with the alluvial, the next clafs of rocks. Thefe 

 are found in the loweit fituations. We have thus, from 

 granite to the alluvial formation, all the feries marked with 

 a diminifhing level, in proportion to the newnefs of the 

 ftrata. 



" Defcription of the Saxon Erzgebirge.— The mine diftrift of 

 the electorate of Saxony has a bafis of granite which rifes 

 through the fuper-incumbent rocks in the different places at 

 Altenberg, Johanngeorgenftadz, and Bobrifch, on the road 

 leading from Freyberg to Drefden. The newer formations, 

 viz. gneifs, mica-flate, topaz-rock, and clay-flate, are 

 wrapped around the granite in mantle-fhaped ftrata, and the 

 diminifhing levels of the outgoings correfpond to the new- 

 nefs of the formations. Over thefe we meet with other pri- 

 mitive formations that overlie the older formations, and their 

 continuity is partly broken, and partly unbroken ; here are 

 porphyry, fienite, newer granite, quartz, and ferpentine. 

 Still lower down we meet with tranfition rocks, of which the 

 lime-ftone appears at Kalk-grun and Wildenfels ; the amyg- 

 daloid at Voghtland ; and the grey-wacke and grey-wacke 

 flate near Freyberg. Still lower down, and often covering 

 the preceding formations, we find fand-ftone and lime-ftone ; 

 and in feveral places, as at Hainchen, Pottchappel near 

 Drefden, and Zwickau, there are depofitions of the coal 

 formation. Laftly, the neweft floctz-trap formation covers 

 all the others in unconformable, overlying, and very broken 

 ftratification." 



Decompo/ition of Rocks. — Where rocks rife above the fur- 

 face, and are expofed to the aftion of air and moifture, they 

 are liable to decompofition and ditintegration. The former 

 confifts in the feparation of the conftituent parts, the latter 

 in the feparation of the integrant parts : the one may be 

 compared to the moulding of the ftoncs of which a building 

 isconltrufted, the other to the disjointing and diflocation of 

 the ftoncs in a found ftate, when a building is thrown down. 

 Both thefe procefl'es frequently take place in the fame 

 rock. 



The difintegration of rocks is fometimes rapidly effc&ed 

 by earthquakes, lightning, and the immediate a&ion of fub- 

 terranean fires. It is, however, to the more conitant ope- 

 rations of moifture and change of temperature, that the 

 dcltru&ion of rocks and mountains may be principally at- 

 tributed ; but no well-authenticated observations have yet 

 been made to determine the extent of tl during a 



given period of time. It has been vaguely Hated, tlrat the 

 height of the Pyrenees is diminifhing about one foot in a 

 century ; hence it was calculated that a million vcars would 

 be required to level the rocky boundary which feparates 

 France and Spain. It is obvious tint a lapfe of many cen- 

 turies would be required to verifj fuch a conclufion ; and 

 though the decompofition and difintegration of rocks are, 

 in many fituations, fufficiently rapid i" fa ible during 



the fhort period of a fingle hie, yet, ift other iituatioDS, 



rockt 



