INTRODUCTION. 



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 con- 

 sidered : first, their nature ; secondly, 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. 



(4.) Next, the general plan or laws of progress in the earth and 

 its life. 



(o.) Finally, there are the active forces and mechanical agencies 

 which were the means of physical progress, — spreading out and con- 

 solidating strata, raising mountains, ejecting lavas, wearing out valleys, 

 bearing the material of the heights to the plains and oceans, enlarging 

 the oceans, destroying life, and performing an efficient part in evolving 

 the earth's structure and features. 



These topics lead to the following subdivisions of the science : — 



I. Physiographic Geology, — a general survey of the earth's 

 surface-features. 



II. Lithological Geology, — a description of the rock-material 

 of the globe, its elements, rocks, and arrangement. 



III. Historical Geology, — an account of the rocks in the 

 order of their formation, and the contemporaneous events in geological 

 history, including both stratigraphical and paleontological geology ; 

 and closing with a review of the system or laws of progress in the 

 globe and its kingdoms of life. 



IV. Dynamical Geology, — an account of the agencies or forces 

 that have produced geological changes, and of the laws and methods 

 of their action. 



