CLASSIFICATION OF STRATA. 1 7 



although, when we find one rock deposited on the eroded 

 surface or upturned edges of another of a much older date, 

 we infer a considerable break in time between the two, yet 

 by looking elsewhere, the missing formations may frequently 

 be discovered. While the British Islands, and even England 

 and Wales, are not without breaks in the succession of rocks 

 comprised within their limits, still they present a very full 

 record of the periods embraced in the history of the earth's 

 crust, and so far as we know furnish a more complete account 

 than does any other tract of similar extent. No doubt our 

 islands have been more carefully and thoroughly investigated 

 than other portions of land ; but the fulness of the record has 

 given great impulse to the study of Geology, and depends on 

 the succession of the strata, and the rarity of great breaks, 

 studied together with the succession of organic remains. 

 Moreover, the physical history of Britain proves that it has 

 been more subject to great changes of level than many other 

 portions of the Earth's surface. 



Nevertheless, it must always be remembered that the 

 geological history of England is not the geological history of 

 the world, any more than the history of the English people is 

 the history of all nations. So long as we base our conclusions 

 on the evidence of changes now in operation, we cannot 

 expect that those which affected England and Wales should 

 have been accompanied by similar changes over a large ex- 

 tent of the earth's surface. Our coal-bearing period was not 

 a coal-bearing period all over the world ; nor is it likely, except 

 perhaps in the earliest geological periods, that the organic 

 species found in our rocks existed over wider areas than do 

 the forms now living around our coasts, and in other regions. 



In order to establish uniformity in the use of the names applied 

 to the strata, the International Geological Commission has recom- 

 mended the adoption of terms in the following order, the most 

 comprehensive being placed first : — 



According to this scheme, we should speak of the Palaeozoic 

 Group or Era, the Silurian System or Period, the Ludlow Series or 

 Epoch, and the Aymestry Stage or Age. 



^ The term Stromatology has sometimes been appHed to the history of the 

 stratified rocks. 



