TEXTURES 189 



The igneous rocks are massive, as distinguished from stratified, 

 and though sometimes presenting a deceptive appearance of strati- 

 fication, may always, with a little care, be readily distinguished 

 from the truly stratified rocks. The term ?nassive is, indeed, 

 frequently used for these rocks, in the same sense as igneous, and 

 eruptive rocks is another term, meaning the same thing, though 

 eruptive is also employed in a more restricted sense. 



Characteristic differences appear between those igneous masses 

 which have solidified deep within the earth and have been brought 

 to light only by the denudation and removal of the overlying 

 rock-masses, and those which have cooled at or near the sur- 

 face of the ground. The former are called plutonic (or intru- 

 sive) and the latter volcanic (or extrusive). Between the two 

 may be found every form of transition, and the terms volcanic 

 and plutonic are now employed for description rather than for 

 classification. 



The texture of an igneous rock means the size, shape, and mode 

 of aggregation of its constituent minerals. Texture is a very im- 

 portant means of determining the circumstances under which the 

 rock was formed, and hence great attention is paid to it. Since 

 texture responds so accurately to the circumstances of solidification, 

 rate of cooling, pressure, etc., all the varieties shade into one 

 another by imperceptible gradations and form a continuous series. 

 Nevertheless, it is necessary to distinguish and name the more 

 important kinds. 



Among the igneous rocks are found four principal types of 

 texture, with several minor varieties. 



1. Glassy. — Here the rock is a glass or slag, without distinct 

 minerals in it, though the incipient stages of crystallization, in the 

 form of globules and hair-like rods, are often observable with the 

 microscope. When the glass or slag is made frothy by the bubbles 

 of escaping steam and gas, the texture is said to be vesicular, 

 scoriaceous, or pumiceous, according to the abundance of the 

 bubbles. These are varieties of the glassy texture, though other 

 kinds may also be vesicular. A vesicular rock in which the steam- 

 holes have been filled up by the subsequent deposition of some 



