CLASSIFICATION OF THE EOZOIC HOCKS OF THE BRITISH ISLES. 291 
Scotland were the oldest in Great Britain, and, as far as we 
know at present, this view seems to be correct. He at first 
associated them with the Laurentian rocks of Canada ; but 
though this correlation is possibly correct, he yet wisely 
grouped them under the local name Lewisian, after the island 
of Lewis, where they are largely developed. Hr. Sterry Hunt 
and myself have at different times pointed out the presence of 
these rocks in Ireland, and Dr. Holl has done the same in the 
Malvern Hills. The special characters which distinguish this 
group may be said to he the prevalence of very highly crystalline 
rocks, usually massive granitic gneisses, with red felspar and 
hornblende as their chief minerals, and quartz and mica in 
small proportion only. The normal strike is about east and 
west, but it varies to north-west and south-east. 
Though hut little additional information in regard to these 
rocks has been recently obtained, we are now better able, in 
consequence of the very important discoveries made of late 
years in other Eozoic rocks of Britain, to realize their 
importance in the classification, as well as the value of the 
formation in relation to the early history of these islands. 
2. Dimetian. 
This formation, like the above mentioned, consists chiefly 
of highly crystalline gneissic rocks, the prevailing types, 
however, being the so-called granitoid rocks, made up largely 
of quartz with some pinkish or white felspar. Hornblende 
is much less abundant than in the Lewisian rocks, but 
mica is more frequently met with. It may be said to con- 
sist chiefly of acid types of rocks, whilst the former is made up 
mainly of basic types. Bands of limestone, hornblende, chlo- 
ritic, and micaceous schists occur occasionally in this formation. 
The name Dimetian was first given by me to the group of 
granitoid and gneissic rocks which had been discovered by the 
late Mr. Salter and myself, underlying the Cambrian rocks at St. 
David’s, in Pembrokeshire. Since then many additional areas of 
these rocks have been found, and the formation has been shown to 
consist of several important groups. In each area certain general 
resemblances in mineral characters are found to prevail, and these, 
combined with the physical evidences of contemporaneous deposi- 
tion, are all the aids we can expect to find in our attempt at cor- 
relating such ancient rocks in different areas. That such evidences 
are important also is clear if we recognize the fact that the rocks 
must have been for the most part deposited in successive layers of 
various materials, like the alternating sediments in more recent 
groups, and in each case under tolerably similar physical con- 
ditions ; that they also must have suffered changes in propor- 
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