﻿186 



DR. W. K. JONES ON THE SECONDARY 



[vol, lxxii r 



(d) Absence of Metamorphism in the ' Glacial ' Clays. 



If the clays and ' boulder-clays ' are of Talchir age, and therefore 

 much older than the Cretaceous granite with which they are in 

 contact, and if they lie, as averred, between metamorphosed lime- 

 stone below and phyllites, schists, and quartzites above, why were 

 they not metamorphosed to the same extent as the rocks between 

 which they lie ? Mr. Scrivenor states that 



' They show no trace of hardening as a whole, or of mineral alteration away 

 from the granite- junction and veins. They might have been quite recently 

 deposited, so far as their physical condition is concerned.' 1 



To account for the absence of metamorphism, he gives two 

 explanations. One is that the clays, owing to their great plas- 

 ticity, escaped metamorphism ; and the other, that, if they did 

 suffer metamorphism, they have now lost all signs of it. It seems 

 quite impossible to reconcile the statement 3 that the absence of 

 regional metamorphism in these clays 



' is probably due to the plasticity of the material acted upon, which enabled 

 the beds to adjust themselves to the strains in the earth's crust, without 

 great development of heat,' 



with his other statement 3 



' that there seems to be a gradual passage upwards from the clays and 

 boulder-clays to the phyllites and quartzites.' 



What evidence is there that the clays below those converted into 

 ph}dlites were so much more plastic than the latter that they were 

 able to escape metamorphism P The only reason given is that the. 

 unmetamorphosed clays are rich in kaolin 4 ; kaolin, however, is 

 characterized as a clay, not by its plasticity, but by its non -plasticity. 

 These unmetamorphosed clays are supposed by Mr. Scrivenor to 

 be of Permo-Carboniferous age, and in that case must have been 

 covered by thousands of feet of strata when the Mesozoic granite- 

 magma, now forming the two granite ranges flanking the valley 

 (and only about 10 miles apart), was intruded. It is inconceivable 

 that any clays, wedged between two subsequently-intruded enormous 

 granite ranges, and lying between beds highly metamorphosed by 

 the granite, could possibly have escaped metamorphism, especially 

 within a few yards of the granite-contact, 5 whatever their nature. 



In support of the alternative explanation that they may have 

 • lost all signs of metamorphism, Mr. Scrivenor draws attention 

 to the following extract from the Rev. W. Howchin's paper on the 

 glacial clays of Australia : — 



' The beds which give evidence of glacial origin may be described as con- 

 sisting mainly of a grand mass of unstratified, indurated mudstone, more or 



1 ' The Geology & Mining Industry of the Kinta District ' 1913, p. 40. 



2 ' The Gopeng Beds of Kinta ' Q. J. G. S. vol. lxviii (1912) p. 146. 



3 ' The Geology & Mining Industry of the Kinta District ' 1913, p. 45. 



4 Ibid. p. 41. 5 Ibid. See geological map. 



