INTRODUCTION 



23 



extent, as a result of the radioactivity of certain minerals, a correct 

 theory of the earth's interior was impossible. The true theory has 

 probably not yet been found, but every advance in knowledge brings 

 the solution nearer. The modification of geological theories from time 

 to time should not be a source of annoyance to the student, but 

 should rather serve to stimulate him to reason for himself. 



Fundamental Terms. — There are a few terms with which the 

 student must become familiar before a discussion of the subjects 

 taken up in the following chapters can be understood. Of these 

 terms only very elementary definitions will be given in this place, 

 since more complete explanations will be taken up later. 



Rocks. — With the exception of a comparatively thin layer of soil, 

 which varies greatly in thickness and is entirely absent in some 

 places, the earth is composed of rock which extends from the surface 

 downward for many miles (the lithosphere), and probably through 

 the central core (the centrosphere). In general, the rocks of the 

 earth's crust can be classified according to their origin as of three 

 kinds: (1) sedimentary, (2) igneous, and (3) metamorphic. 



Sedimentary Rocks. — If one examines the sediment deposited by 

 a muddy rivulet in a temporary pool of water, he will find that it 

 consists of sand or clay, and that it is in layers. This deposition 



Fig. 1. — Niagara limestone, showing well-bedded layers with two sets of strong 

 joints at right angles to each other. (U. S. Geol. Surv.) 



