ELEMENTARY FACTS. 



57 



CHAPTER V. 



ELEMENTARY FACTS AND PRINCIPLES OF GEOLOGY. 



Our Knowledge of the Internal Structure of the Earth very- 

 Limited. — Variety of Minerals and Rocks, arranged in a de- 

 terminate Order. — Advantages of Geological Knowledge. — 

 Division of Rocks : Stratified, Unstratified, Parallel, In- 

 clined, Cropping out, Dip, Thickness, Outliers, Escarpment, 

 Faults, Mineral Veins, Rock Dike, Clay Dike, Formations. — 

 Illustrations. — How to Observe. — Valleys of Denudation, &c. 



Having considered the properties of matter, the 

 chemical and mineralogical constitution of the 

 earth's surface, and some points connected with 

 its physical geography, we are now prepared to 

 contemplate those general truths, obtained by obser- 

 vation and experiment, concerning the arrangement 

 of these materials, so far as they may be open to 

 our inspection. We at once, however, perceive 

 that our investigation must necessarily be confined 

 to a comparatively small portion of the mass of the 

 earth, namely, its superficial external crust, which 

 tjie efforts of man have hitherto failed to penetrate 

 to a greater depth than about 3000 feet. To this 

 we may add the elevation of the highest mountains, 

 and then we find our direct knowledge is confined 

 within the limits of about one eight hundredth part 

 of the earth's semi-diameter ; a perpendicular dis- 

 tance varying from five to eight or ten miles ; bear- 

 ing a proportion to the mass of the earth less than 

 that of a thin coat of varnish to an artificial globe 

 of two feet diameter. One of the first things which 

 strikes our observation, as we begin to notice the 

 materials of which this crust is composed, is, that 

 they do not form a simple homogeneous mass, con- 



