CHRONOLOGICAL SUCCESSION OF AQUEOUS ROCKS. 1 1 



special and characteristic assemblages of animals and plants ; 

 but, in a general way, each subdivision of each formation has 

 its own peculiar fossils, by which it may be recognised by 

 a skilled worker in paleontology. Whenever, for instance, 

 we meet in Britain with the fossils known as Graptolites, we 

 may be sure that we are dealing with Silurian Rocks. We 

 may, however, go much further than this. If the Graptolites 

 belong to certain genera, we may be sure that we are dealing 

 with Lower Silurian Rocks. Furthermore, if certain special 

 forms are present, we may be even able to say to what exact 

 part or subdivision of the Lower Silurian series they belong. 



All these conclusions, however, would have to be accom- 

 panied by a tacit but well-understood reservation. No Grap- 

 tolites have ever been found in Britain out of rocks known 

 upon other grounds to be Silurian ; but there is no reason why 

 they might not at any time be found in younger deposits. In 

 the same way, the species and genera which we 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 palaeontological evidence alone, that we are always reason- 

 ing 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 ROCKS. 



, 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 uniformity 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, characterised and 

 distinguished in the same way. It is not pretended for a mo- 

 ment 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 formations can be seen to- 

 gether; 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 succession. The reason of 

 this is obvious. There are many places to take a single ex- 

 ample where one may see the Silurian Rocks, the Old Red 

 Sandstone, and the Carboniferous Rocks succeeding one an- 



