Descriptive Arrangement of Volcanic Rocks. 35 



and by this character the sub-species may be, with proprie- 

 ty, distinguished. 



All lithoidal volcanic rocks, with the exception only of 

 those which have partly, or wholly, passed by complete fu- 

 sion into the state of glass, consist of an aggregation of more 

 or less imperfect crystals of one or more minerals. 



The average size of these crystals, or integrant particles, 

 determines the grain of the rock, which is one of the ele- 

 ments of its texture. When the average size of the crystals 

 is so large as to strike the eye by its crystalline structure at 

 a distance, as in granites, the texture is called granitic ; 

 when of such size as to be discerned only by close inspection, 

 granular ^ and when so minute as to require a lens to ascer- 

 tain its crystalline texture, or the assistance of the mechani- 

 cal analysis, compact. When the rock appears to be passing 

 to the state of a glass or enamel, assuming a pearly, waxy, 

 or resinous lustre, its texture is called resinous, or semi-vitre- 

 ous ; and lastly, the finest texture of all is the vitreous, or 

 glassy. 



But, besides the size of the crystalline particles, another 

 character influences the texture of the rock, viz., their more 

 or less intimate aggregation, which may be loose and inco- 

 herent, giving an earthy aspect to the rock ; or close and 

 compact, producing the effect of hardness and solidity. 

 Another and still more important characteristic is, the regu- 

 lar or irregular disposition of its component crystalline parti- 

 cles, which are sometimes aggregated in a confused and dis- 

 orderly manner, without any determined method, so as to 

 give an irregular fracture to the rock, as in granite, clay- 

 stone, &c. ; at others, they are arranged so that their long- 

 est plane surfaces preserve a more or less perfect parallelism 

 to one another, through a considerable space, by which a 

 foliated, or scaly texture is given to the rock, and a splintery 

 or slaty fracture, as well as a lamellar or schistose structure, 

 on a large scale. This remarkable difference in the diposi- 

 _ tion of the crystalline particles is always found to pervade 

 the whole mass of rock, and, in fact, forms the only distinc- 

 tion between granite and gneiss, claystone and clinkstone. 



Hence, according to the arrangement proposed here, the 

 sub-species of the volcanic rocks should be distinguished by 

 epithets significative of their peculiar texture, viz. 



