MECHANICAL DEPOSITS IN MOUNTAINS, &C. \& 



&c. from Ardnamurclmn to Glenelg, confifts, in a great mea" 

 fare, of a granitic fandftone, in vertical beds. This ftone 

 fometimes occupies great tracts ; at other times, it is alter- 

 nated with the micaceous, and other varieties of primary 

 fhiftus ; it occurs, likevvife, in feveral of the iflands, and is a 

 foffil which we hardly find defcribed or named by writers on 

 mineralogy." 



The granitic fandftone of Glen-elg is mod certainly gneifs *, Ofcf. This gra- 

 and a variety which is not uncommon ; and I may venture to n ^ f andftonci« 

 fay, that the ftrata in Arafaig, &c. are of the fame nature. 



Profeflbr Play fair concludes with dating the following exam- Other example* 



pies, as a further confirmation of his opinion — •' Much alio of ?! r * J J ' a ^/ air * 

 1 / .... Granulated 



a highly indurated, but granulated quartz, is found in feveral quartz in beds 

 places in Scotland, in beds of ftrata, alternated with the com- a ; tern atingwit* 

 inon lhilius or the mountain. Remarkaole inftances or this 

 may be feen on the north fide of the ferry of Balachulifh, and 

 again on the fea fhore at C alien. At the latter, the ftrata are 

 remarkably regular, alternating with different fpecies of fhiftus. 

 At the former, the quartz is fo pure, that the ftone has been 

 miftaken for marble, 



'!. Thefe examples are perhaps fufficient ; but I muft add, 

 that in the micaceous and talcofe fchifti themfelves, thin layers 

 of fan d are often found interpofed between the layers of mica 

 or talc. I have feen a fpecimen from the fummit of one of the 

 higheft of the Grampian mountains, where the thin plates, of 

 a talcy or aibeftine fubftance, are feparated by layers of a 

 very fine quartzy fand, not much confolidated. The moun- 

 tain from which it was brought, confifts of vertical ftrata, 

 much interfered by quartz veins. It is impoftible to doubt, Whence he 

 in this inftance, that the thin plates of the one fubftance, and concludes that 

 the fmall grains of the other, were depofiled together at the f, te( j (, ne chanx-* 

 bottom of the fea, and that they were alike produced from caIJ y) at th « 

 the degradation of rocks more ancient than any which now d m * 

 exiir." 



I am furprized Profeflbr Playfair mould adduce granular 0bf. Granular 

 quartz as a proof of mechanical depoiition, as it has no more T tiartz \ s not * 

 claim to fuch a character than granite. The following obfer- a chemical depo- 

 rations will render this evident; granular quartz differs from fition \ 

 mica flate, in the abfence of the flaty fraclure, and mica ; we f rom ^ica fhr* 



in the abfence of 

 # Mineralogy of the Scottish ifles, vol. ii. p. 160. flaty fracture* 



l^ye but the feris» 



