Descriptive Arrangement of Volcanic Rocks. 35 
and by this —— the sub-species may be, with proprie- 
vn lithoidal volcanic rocks, with the exception only o 
those which have partly, or evil, passed by complete a 
sion into the state of glass, consist a an aggregation ‘of more 
or less imperfect crystals of one or more mine 
The average size of these crystals, or ‘itegrant particles, 
determines the grain of the rock, which is one of the ele- 
ments of its texture. When the average size of the crystals 
~ SO mi as to strike the eye ae its a structure at 
in granites, the is called granitic ; 
vies of sacks size as to be aiecormae nif 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 
lane besides the size of the a rere particles, another 
character influences the texture of the , Viz., their more 
gi eart 
compact, producing the 
Another and still more importa characteristic is, the regu- 
the whole m rock, and, in fact, forms the onl 
tion between granite and gneiss, claystone e and ne. 
Hence, according to the arrangem d here, the 
ite aly of the volcanic rocks Should be ee by 
ts significative of their peculiar texture, viz: 
