ROCKS, ROCK^WEATHERING, 



AND SOILS 



PART I 



THE CONSTITUENTS, PHYSICAL AND CHEMICAL 

 PROPERTIES, AND MODE OP OCCURRENCE OF 

 ROCKS 



L INTRODUCTORY 



A ROCK is a mineral aggregate; more than this, it is an 

 essential portion of the earth's crust, a geological body occu- 

 pying a more or less well-defined position in the structure of 

 the earth, either in the form of stratified beds, eruptive masses, 

 sheets or dikes, or in that of veins and other chemical deposits 

 of comparatively little importance as regards size and extent. 

 In giving this definition, origin, chemical composition, and state 

 of aggregation of the individual particles are for the time 

 ignored. From a strictly geological standpoint, the beds of 

 loose sand, and even the water of the ocean itself, may be 

 considered as rocks, and either, under favorable circumstances, 

 may undergo a process of induration such as shall be produc- 

 tive of the condition of solidity commonly ascribed to rocks 

 by the popular mind. 



In ever-varying conditions as regards compactness, color, 

 texture, and structure, rocks form the entire mass of the globe 

 so far as it is as yet made known to us, with the exception of a 

 scarcely appreciable proportion of organic matter. It is rock 

 which forms the substance of mountain ranges and the vast 

 stretches of valley and plain. It is from the rocks that we 

 gain our food, our fuel, and the supplies of metal which are 

 seemingly so essential to our well-being; we cannot ignore 

 2 1 



