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POPULAR SCIENCE REVIEW. 
has been used for some time by American authors for the rocks 
known under the several names of Pre- Cambrian, Archaean, or 
Primary. Possibly none of the names hitherto suggested can 
remain in the face of future discoveries; but as long as the 
term Palceozoic continues to be so universally used for the suc- 
ceeding period, the term Eozoic is probably the best that can 
at present be applied to this more ancient one. 
The Eozoic rocks in Britain have of late years received a 
considerable amount of attention, and much additional informa- 
tion concerning them has in consequence been obtained. Up to 
a recent period it was supposed that with the exception of a 
narrow strip along the north-west coast of Scotland and in the 
Hebrides, there was no satisfactory evidence of the presence of 
these primary rocks anywhere in Britain. Stratified crys- 
talline rocks had been noticed in other areas, but these were 
almost universally placed among much more recent deposits, 
either on theoretical grounds or by misconceptions in strati- 
graphy. "With the exception of careful stratigraphical work 
nothing has tended so much of late to advance our knowledge in 
regard to these early rocks, as the recognition of the fact that the 
metamorphism of great groups of strata does not take place so 
readily as was formerly supposed, and that it is now tolerably 
clear that some special conditions, such as do not appear to have 
prevailed to any great extent over the British areas since the Pre- 
Cambrian (Eozoic) period, were necessary to produce such results. 
The unravelling of the history of these primitive rocks is 
fraught with many difficulties, and in the absence of organic 
remains the correlation of the groups in different areas is 
seldom as satisfactory as could be desired. But that there are 
special characters present which tend to distinguish each im- 
portant group over wide areas there can be no doubt ; and 
when we look closely into these, and at the same time take 
note of any influence which local physical conditions may 
have exerted, we can generally arrive at a tolerably satisfactory 
correlation. 
We now divide the Eozoic rocks which are exposed in the 
British areas into four distinct formations under the names of 
Lewisian, Dimetian, Arvonian, and Pebidian. 
1. Lewisian. 
The oldest stratified rocks apparently in this country are, 
some gneiss rocks, found chiefly in the Hebrides and along tke | 
north-west coast of Scotland, also probably along the north-; 
west coast of Ireland, and perhaps in the Malvern Hills inj 
England. Many years ago Sir B. Murchison expressed the 
opinion that these gneiss rocks along the north-west coast oft 
