PAET III. 



HOCKS AND ROCK-PRODUCING AGENCIES. 



I. GENERAL CLASSIFICATION OF ROCK MASSES. 



The term " rock " in its geological acceptation, includes all the 

 stony and earthy masses whether consolidated, as granite, lime- 

 stones, &c., or composed of loosely coherent particles, as sands and 

 gravels which make up the outer or visible portion of the earth. 

 The mean radius of the earth-mass, or distance from centre to surface 

 is equal to 3956 miles. The elevations and depressions which occur 

 upon the earth's surface, forming mountain-chains and table-lands, 

 valleys and the beds of seas and lakes, are thus, as compared with 

 this radius, of but slight significance. It is necessary to bear this in 

 mind, in order that we may not exaggerate the intensity of the 

 forces by which these inequalities have been produced. In a section 

 or profile in which the same scale is employed for longitudinal and 

 vertical dimensions, the greatest inequalities become scarcely appar- 

 ent. In order to render evident the differences of level existing 

 between separate points, it is necessary in engineering drawings, 

 and in ordinary geological diagrams, to use a greatly exaggerated 

 scale for heights or depths as compared with horizontal distances ; 

 and the eye unconsciously follows a somewhat similar process in 

 taking in the contour-lines or general aspect of a mountainous region. 



Our knowledge of the internal condition of the earth is necessary 

 to a great extent conjectural but the weight of evidence, collected 

 in reference to this subject, leads to the conclusion that the earth- 

 mass, from surface to surface, is not throughout a perfectly solid 

 body. In the opinion of some investigators, the central portion is 

 solid, and between this and the consolidated surface-layers a zone of 

 fluid or incandescent matter exists. According to others, the earth- 

 mass is more or less consolidated throughout, but with enormous 



