30 INTRODUCTION. 



now regard as characteristic of the Lower Silurians, might at 

 any time be found to have survived into the Upper Silurian 

 period. We should never forget, therefore, in determining 

 the age of a rock by palasontological evidence alone, that 

 we are always reasoning upon generalisations which are the 

 result of experience alone, and which may at any time be 

 overthrown by fresh discoveries. 



CHRONOLOGICAL SUCCESSION OF THE AQUEOUS EOCKS. 



As the result of observations made upon the superposition 

 of rocks in different localities, from their mineral characters, 

 and from their included fossils, geologists have been able to 

 divide the entire stratified series into a number of different 

 divisions or formations, each characterised by a general uni- 

 formity of mineral composition, and by a special and peculiar 

 assemblage of organic forms. Each of these primary groups 

 is in turn divided into a series of smaller divisions, charac- 

 terised and distinguished in the same way. It is not pre- 

 tended for a moment that all these primary rock-groups can 

 anywhere be seen surmounting one another regularly. There 

 is no region upon the earth where all the stratified forma- 

 tions can be seen together; and, even when most of them 

 occur in the same country, they can nowhere be seen all 

 succeeding each other in their regular and uninterrupted suc- 

 cession. The reason of this is obvious. There are many 

 places to take a single example where one may see the 

 Silurian Eocks, the Old Eed Sandstone, and the Carboniferous 

 Eocks succeeding one another regularly, and in their proper 

 order. This is because the particular region where this occurs 

 was always submerged beneath the sea while these formations 

 were being deposited. There are, however, many more local- 

 ities in which one would find the Carboniferous Eocks resting 

 unconformably upon the Silurians without the intervention of 

 any strata which could be referred to the Old Eed Sandstone. 

 This might arise from one of two causes : 1. The Silurians 

 might have been elevated above the sea immediately after 

 their deposition, so as to form dry land during the whole of 

 the Old Eed period, in which case, of course, no strata of the 



