302 REPORT o.\ NATIONAL MUSEUM, 188G. 



C.— PHYSICAL AND CHEMICAL PROPERTIES OF ROCKS. 



A little space may be well devoted here to a consideration of those 

 properties of rocks which can be grouped under the heads of density? 

 hardness, and structure, together with notes on their color and chemical 

 composition. 



(1) DENSITY AND HARDNESS. 



Density. — This is an important property, since upou it are dependent 

 to a large extent the weight per cubic foot, the strength, and the absorp- 

 tive powers of the stone. Among rocks of the same mineral composi- 

 tion, those which are the densest will be found heaviest, least absorptive, 

 and usually the strongest. 



To ascertain the weight of a rock it is customary to compare its 

 weight with that of an equal bulk of distilled water, in other words to 

 ascertain its specific gravity. The specific gravity multipled by 02.5 

 pounds (the weight of a cubic foot of water) will thus give the weight 

 per cubic foot of stone. The weights given in the tables have been thus 

 computed. (See p. 000.) 



Hardness. — The apparent hardness of a rock is dependent upon (1) the 

 hardness of its component minerals and (2) their state of aggregation. 

 However hard the minerals of a rock may be, it appears soft and works 

 readily if the particles adhere with slight tenacity. Many of the softest 

 sandstones are composed of the hard mineral quartz, but the grains fall 

 apart so readily that the stone is as a whole soft. (See under State of 

 Aggregation.) 



(2) STRUCTURE. 



Under this head are considered those characters of rocks which are 

 dependent upon the form, size, and arrangement of their component 

 minerals. 



All rocks may be classified sufficiently close for present purposes 

 under one of the three heads (1) crystalline, (2) vitreous or glassy, and 

 (3) fragmental. Of the first, granite and crystalline limestone may be 

 considered as types ; of the second, obsidian and pitchstone, and of the 

 third, sandstone. Many structural properties are common to all, others 

 are confined to rocks of a single type. Accordingly as the structure is 

 or is not readily recognizable by the unaided eye, we have : 



(1) Macroscopic structure, or structure which is distinguishable in the 

 hand specimen and without the aid of a microscope. — Under this head are 

 comprehended structures designated by such names as granular, mass- 

 ive, stratified, foliated, porphijritic, concretionary, etc.; terms whose 

 precise meaning is given in the glossary, aud which, with perhaps one 

 or two exceptions, need not be further considered here; aud 



(2) Microscopic structures. —Many rocks are so fine grained and com- 

 pact that nothing of tlieir mineral nature or structure can be learned 

 from study with the eye alonoj and recourse must be had to the micro* 



