Br. C. Callaimy — How to Work the Archman EocJcs. 423 



But a careful study of the whole district leads to the conclusiou 

 that these altered patches are subordinate parts of a conijDaratively 

 unaltered group which, taken as a whole, differs widely from the 

 moie ancient series, which is thoroughly metamorphosed from top 

 to bottom. 



In studying the lithology of a formation, its degree of metamorpMsm 

 is an important factor in the evidence. Sufficient material has not 

 yet been collected to form the basis of a theory ; but, so far at least 

 as England and Wales are concerned, the researches of the last few 

 years lend some support to the opinion that regional metamorphism 

 is found only in Archaean rocks, and that the degree of alteration 

 is pro]3ortioned to the antiquity of the group. Murchison held that 

 the Malvern metamorphic rocks were altered Cambrian, but no one 

 now doubts their Archsean age. The "altered Cambrian," mapped 

 by the Geological Survey as forming a band flanking each side of 

 the granitoid axis of St. Davids, clearly underlies the basement 

 conglomerates of the Lower Cambrian, and is the typical Pebidian 

 of Hicks. The " altered Caradoc " which is found here and there 

 in the central Salopian chain is sometimes an unaltered rock, some- 

 times a quartzite whose Archaean age cannot be disproved, but 

 generally it is a part of the great Archeean volcanic series. The 

 altered rocks of Anglesey, the " metamorphic Cambrian and Silurian " 

 of the Survey, are shown by a variety of evidence to be older than 

 the Cambrian. Rocks of precisely the same mineral character, 

 showing the same order of succession, and lying on the same strike, 

 are sometimes coloured by the Survey as " Cambrian," sometimes as 

 " Silurian." True Cambrian and " Silurian " strata do indeed 

 occur in many parts of Anglesey, and they have given their name 

 on the map to the metamorphic rocks which happened to be near 

 them ; but in no case are there any signs of a transition between 

 the altered and unaltered beds. The evidence, then, on which 

 the Cambrian and " Silurian " age of the metamorphic rocks of 

 Anglesey has been based proves, on a critical examination, to be 

 unsound, and indeed, in some cases, self-destructive. Along the 

 north-western margin of Caernarvonshire are bands of supposed 

 "altered Cambrian," which in like manner turn out to be of greater 

 antiquity. The partially altered rocks of the Charnwood Forest 

 area, regarded as Cambrian by the Survey, are now, as previously 

 shown, considered to be Pebidian. Every case of supposed meta- 

 morphic Cambrian or " Silurian " has been invalidated by recent 

 researches, and we are driven to the conclusion that within the 

 English and Welsh area there is a presumption in favour of the 

 supposition that any district of altered rock which may be discovered 

 is of Archgean age. 



That the degree of metamorphism is proportioned to the antiquity 

 of a rock-group is strongly suggested in England and Wales. The 

 Pebidian of St. Davids has undergone chemical change, but has not 

 been converted into a crystalline schist. Shales have become 

 chloritic, felspathic mud has been changed to hornstone, and vol- 

 canic ash has been indurated into a rock bearing a superficial re- 



