2 INTRODUCTORY. 



geognosy. 2. Dynamical geology, or physical and chemical geology. 

 3. Historical geology, or the history of the earth. 



Bat there are two important points of difference between geology 

 and organic science. The central department of organic science is 

 physiology, and both anatomy and embryology are chiefly studied to 

 throw light on this. But the central department of geology, to which 

 the others are subservient, is history. Again : in case of organisms 

 — especially animal organisms — the nature of the changes producing 

 development is such that the record of each previous condition is suc- 

 cessively and entirely obliterated ; so that the science of embryology is 

 possible only by direct observation of each successive stage. If this 

 were true also of the earth, a history of the earth would, of course, be 

 impossible. But, fortunately, we find that each previous condition of 

 the earth has left its record indelibly impressed on its structure. 



Order of Treatment. — The prime object of geology is to determine 

 the history of the earth, and of the organisms which have successively 

 inhabited its surface. The structure and constitution of the earth are 

 the materials of this history, and the physical and chemical changes 

 now going on around us are the means of interpreting this structure 

 and constitution. Evidently, therefore, the only logical order of pre- 

 senting the facts of geology is to study, first, the causes, physical and 

 chemical, noiwin operation and producing structure ; then the structure 

 and constitution of the earth which, from the beginning, have been 

 produced by similar causes ; and, lastly, from the two preceding to un- 

 fold the history of the earth. 



Geology may be defined, therefore, as the history of the earth and 

 its inhabitants, as revealed in its structure, and as interpreted by 

 causes still in operation. 



There is no other science which requires for its full comprehension 

 a general knowledge of so many other departments of science. A 

 knowledge of mathematics, physics, and chemistry, is required to under- 

 stand dynamical geology ; a knowledge of mineralogy and lithology is 

 required to understand structural geology ; and a knowledge of zoology 

 and botany is required to understand the affinities of the animals and 

 plants which have successively inhabited the earth, and the laws which 

 have controlled their distribution in time. 



