OP CREATION. 13 



The remaining subdivisions are often very strong- 

 ly, though not so completely indicated; and to these I 

 shall apply the name " Period" as also sufficiently 

 convenient. It will be found that the different epochs 

 and periods described are in most cases distinguished 

 by a commencement and a termination, often not the 

 less interesting that each exhibits an occasional pas- 

 sage, both by mineral structure and fossil remains, into 

 the beds of the next succeeding one. 



In describing the groups of fossils, however, it will 

 be necessary, in order to avoid repetition, that we 

 should as far as possible confine our attention in each 

 case to some group of animals or vegetables whose 

 remains are most characteristic of the particular 

 period which they are assumed to illustrate ; and for 

 the sake of convenience we shall often perhaps seem 

 to neglect, or pass by with very slight mention, those 

 which are nevertheless widely distributed in the rocks 

 of the period under consideration. This might lead 

 to some confusion, and even to wrong conclusions, if 

 it were not understood beforehand that such apparent 

 neglect is not without a reason. 



In order to remedy this evil in some measure, I 

 have here appended a tabular view of the various pe- 

 riods, in the order in which they will be treated, and 

 with particular reference to the forms of organic life 

 most strikingly exhibited in each. By glancing the 

 eye over this table, the reader, however little ac- 

 quainted with the details of Geology, will at least be 

 enabled to recognize the plan, and will thus enter on 

 the descriptions with some general notion of their 

 bearing on the whole range of creation. 



