14 GEOLOGY. 



has ever failed to find it. It may, therefore, be accepted as a fact 

 that the upper surface of the sohd rock is nowhere far below the surface. 



Concerning the thickness of the crust, if there be any true crust 

 at all, little is known by direct observation. The deepest valleys, 

 such as the canyon of the Colorado, and the shafts and borings of the 

 deepest mines and wells, give knowledge of nothing but rock. The 

 deepest excavations extend rarely more than a mile below the surface. 

 It is certain that rock of known kinds extends to far greater depths. 



The interior. — Concerning the great interior of the earth, little is 

 known except by inference. From the weight of the earth, ^ it is in- 

 ferred that its interior is much more dense than its surface. From 

 its behavior under the attraction of other bodies, it is believed to be at 

 least as rigid as steel, and its interior cannot, therefore, be liquid, 

 in the usual sense of that term. From the phenomena of volcanoes, 

 and from observations on temperature in deep borings, it is inferred 

 that its interior is very hot. Further inferences concerning its char- 

 acter are less simply stated, and will be referred to later. 



The solid part of the earth is therefore composed of (1) a thin layer 

 of unconsolidated or earthy material, a few feet to a few hundred feet 

 in thickness, covering (2) a layer or zone many thousands of feet, and 

 probably many miles, thick, composed of solid rock comparable to 

 that exposed at numerous points on the surface, and (3) a central mass, 

 to which the preceding layers are but a shell, composed of hot, dense, 

 and rigid rock, the real nature of which is not known by observation. 



Varieties of rock in crust. — If the mantle of soil, subsoil, and gla- 

 cial rubbish were stripped from the land, the surface beneath would 

 be found to be made up of a great variety of rocks, all of which may be 

 grouped into two great classes. About four-fifths of the land surface 

 would be of rock arranged in layers, and the other fifth would be of 

 crystalline rock, generally without distinct stratification, and often 

 bearing evidence of the effects of high temperature. 



Stratified rocks. — The composition of most stratified rocks corre- 

 sponds somewhat closely with the composition of sediments now being 

 carried from the land and being deposited in the sea. Their arrange- 

 ment in layers is the same, and the markings on the surfaces of the 

 layers, such as ripple- marks, rill-marks, wave-marks, etc., are identical. 

 Furthermore, the stratified rocks of the land, like the recent sediments 



* Its specific gravity as a whole is about 5.57, and the specific gravity of its outer 

 portion is about 2.7. , 



