tRANSACTTONS OF SECTtO^ f1. 705 



12. The Derivation of Sand and Clay from Granite. 

 By Professor W. Boyd Uawkins, D.Sc, F.R.S. 



The decomposition of granite by the attack of the carbonic acid in the rain-water 

 on the soluble crystalline elements of granite results in the formation of a surface- 

 covering more or less complete over the solid rock which can only be studied 

 in non-glnciated regions. It is conspicuous by its absence from the ice-swept 

 granite areas of the Lake country, of Scotland, and of Ireland, and of Middle and 

 Northern Europe. 



The quartz in the granite has resisted decomposition, and where the finer 

 products of decomposition have been swept away form a coarse sand, each grain 

 presenting an irregular surface indented by the feldspars and micas as thoy cooled 

 from the heated magma. These are traceable more or less through a lai'ge 

 number of sandstones, and more especially through those of the Millstone Grits 

 and Coal-measures of Middle and Northern England. 



The attack of the rain-water containing carbonic acid on the micas results in 

 the decomposition of the biotite and to a lesser degree of the muscovite, while 

 the soluble feldspars, such as orthoclase, are completely dissolved, constituting 

 hydrated silicates of alumina and new minerals such as kaolinite and secondary 

 minutely crystalline muscovite. All these occur in the china clay of Cornwall 

 and Devon, and are invariably associated with grains of quartz, primary mica, 

 and tourmaline present in the unaltei'ed granite. All these elements occur in 

 all the samples ranging from the purest china clay through the whole series 

 which have as yet been examined, with the addition of others of local derivation. 



These facts indicate that granite has been one of the chief sources, not merely 

 of the arenaceous, but also of the argillaceous rocks. It is not improbable that 

 both may ultimatel'^^ be proved to have been derived from the siliceous acid layer 

 believed by Dnrocher and Haughton to have been the first to become solid in the 

 cooling globe. 



1:^. Interim Jieport on the Microseopical and Chemical Compositioii of 



Charmvood Bocks. 



MONDAY, SEPTEMBER 7. 

 The following Papers and Reports were read : — 



1. Glacial Erosion in North Wales, By Professor W. M. Davis. 



The mountains of the Snowdon district are believed to represent a group of 

 monadnocks which surmounted the peneplain to which a large part of the region 

 was reduced in Tertiary time. The valleys between the monadnocks were some- 

 what deepened by normal erosive processes, in consequence of a general elevation 

 of the region in late Tertiary time. As a result, the topography of the Snowdon 

 district in immediately pre-glacial time may be described as exhibiting a group of 

 well-subdued mountains, drained through valleys of somewhat sharpened form'. The 

 difference between the forms thus described and the forms seen to-day in the 

 Snowdon district is very great, both in amount and in kind, and cannot be 

 accounted for by normal erosion during glacial and post-glacial time. But the 

 difference is, in amount aud kind, just what might result from glacial action, if it 

 be postulated that glaciers are effective eroding agencies. The depth of glacial 

 erosion in certain cwms and valleys is believed to have been 400, GOO, or 800 feet ; 

 the breadth of glacial erosion must have been of even greater measure. 



1908. 



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