IV. THE PHYSICAL AND CHEMICAL PROPERTIES 



OF ROCKS 



1. STRXTCTITRE 



In considering the structure of rocks it will facilitate mat- 

 ters to do so under two heads: (1) the macroscopic (or mega- 

 scopic) structures, or structures visible to the unaided eye 

 {macros^ from Greek word /xaxpo?, signifying large) ; and (2) 

 microscopic structures, or those visible only with the aid of the 

 microscope. 



1. Macroscopic Structures. — From a structural standpoint 

 all rocks may be classified closely enough for present purposes, 

 under the heads of: (1) Crystalline, (2) vitreous or glassy, 

 (3) colloidal, and (4) clastic or fragmental, Of the first of 

 these, ordinary granite or crystalline marbles are good types, 

 being made up wholly of crystal aggregates, without interstitial 

 amorphous or fragmental material. The term crystalline gran- 

 ular, or granular crystalline, is applied to such as have a dis- 

 tinctly granular structure, as do many of the granitic rocks. 

 Vitreous or glassy structures are found only among igneous 

 rocks, and are due to a cooling of the molten magma too rapidly 

 for the production of crystals. Obviously, as the rate of cooling 

 in rock masses must be extremely variable, so one iSnds all 

 intermediate stages between the completely glassy and the crys- 

 talline forms. To these intermediate stages such names as felsitic 

 and microhtic are given, the precise meaning of which will be 

 stated under the head of microscopic structures. Rocks origi- 

 nating as chemical deposits, and which have since undergone no 

 structural changes, often present a jelly or glue like structure 

 known as colloidal. Such are exemplified in the siliceous sinters 

 from the Yellowstone National Park, and by various other 

 forms of silica, and occasionally by serpentines. 



A clastic or fragmental structure is found only in secondary 

 rocks, and is the result of a breaking down or disintegration of 

 pre-existing rocks, and a reconsolidation of their particles with- 

 out crystallization. There are many minor points of structure, 



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