INTRODUCTION. 13 



succession occupied the land and waters. The history is a history as com- 

 plete as can be learned from the fossils of the life of the globe, as well 

 as of its rock-formations ; and the life-history, imperfect though it be, is 

 the great topic of Geology : it adds tenfold interest to the other records of 

 the rocks. 



These examples are sufficient to explain the basis and general bearing of 

 geological history. 



The method of interpreting the records rests upon the simple principle 

 that rocks were made as they are now made, and life lived in olden time as 

 it now lives ; and, further, the mind is forced into receiving the conclusions 

 arrived at by its own laws of action. We observe that many of the common 

 rock-strata consist of the same materials that make up the deposits of sand 

 and gravel of sea-beaches or sand-fiats, or of the clays or muds of the bottoms 

 of estuaries or the borders of rivers, and that they are arranged in beds like 

 the modern deposits, even have, at times, ripple-marks and other evidences 

 of the action of water or wind ; and further remark that these hard rocks 

 differ from the loose sand, clay, or pebbly deposits simply in being consoli- 

 dated into a rockj and, in other places, discover these sand-deposits in all 

 states of consolidation, from the soft, movable sand, through a half-compacted 

 condition, to the gritty sandstone. By such steps as these, the miud is 

 borne along irresistibly to the conclusion that rocks were slowly made 

 through common-place operations. 



These few examples elucidate the mode of reasoning upon which geo- 

 logical deductions are based. 



In using the present in order to reveal the past, we assume that the 

 forces in the world are essentially the same through all time; for these 

 forces are based on the very nature of matter, and could not have changed. 

 The ocean has always had its waves, and those waves have ever acted in 

 the same manner. Running water on the land has ever had the same power 

 of wear and transportation and mathematical value to its force. The 

 laws of chemistry, heat, electricity, and mechanics have been the same 

 throughout time. The plan of living structures is fundamentally one, for 

 the whole series belongs to one system, as much almost as the parts of an 

 animal to one body ; and the relations of life to light and heat, and to the 

 atmosphere, have ever been the same as now. The laws of the existing 

 world, if perfectly known, are consequently a key to past history. 



SUBDIVISIONS OF GEOLOGY. 



(1) Like a plant or animal, the earth has its systematic external form 

 and features, which should be reviewed. 



(2) Next, there are the constituents of the structure to be considered: 

 first, their nature; second, their general arrangement. 



(3) Next, the successive stages in the formation of the structure, and 

 the concurrent steps in the progress of life, through past time. 



