CHAPTEE I. 



TIME. 



BELOW the oldest rocks which contain definite organic 

 remains there are in certain parts of the British 

 Islands still older rocks, which are now generally known 

 by the name of Archaean, as being the most ancient rock- 

 masses that have yet been recognized in the earth's crust. 

 They are also called Pre-Cambrian, for their infra-position 

 to the Cambrian rocks is the real proof of their antiquity. 



In England, the principal Archaean tracts occur in South 

 Wales (Pembrokeshire), North Wales (Carnarvon and 

 Anglesey), Shropshire (Wrekin and Caradoc range), Charn- 

 wood Forest, the Malvern Hills, and the Lizard peninsula. 

 In Scotland, the Hebrides, as well as parts of Eoss and 

 Sutherland, consist of Archaean rocks. In Ireland, they 

 occur in Donegal and G-alway. 



The Archaean rocks are all more or less metamorphic ; 

 some are distinctly crystalline and foliated, others are as 

 clearly volcanic ashes and lavas, while some seem to be of 

 sedimentary origin, such as the quartzites and limestones. 

 The general opinion is that these Pre-Cambrian rocks are 

 divisible into two great groups, 1 which may or may not 

 belong to distinct systems. The older group consists 

 chiefly of gneiss, which is often so granitoid as to resemble 

 a true granite ; the newer group is composed either of vol- 



1 Callaway, " Geol. Mag.," Dec. 3, vol. ii. p. 258. 



