CRYSTALLINE STRUCTURES OF ROCKS. 185 



contrary, sufficient time be granted, the consequence 

 is the generation of numerous minerals, producing, 

 not only the granitic structure, but the porphyritic 

 also. It is not necessary here to argue the question 

 of graphic granite, which was originally brought for- 

 ward to prove the same conclusion ; since the basis 

 of the reasoning is the same, arid the nature of that 

 structure is particularly described under the head of 

 granite. (Chap, xxiv.) 



The last structure to be noticed is the amygdaloidal, 

 and it is convenient to examine it here, that the whole 

 of the subject of structure, as far as it forms an object 

 of geological theory, should be seen in one condensed 

 view. If the explanation of its cause here given be 

 admitted, it will however be seen that it has no proper 

 title to rank among the modifications of the concre- 

 tionary structure. 



This structure is limited to the trap family and to 

 the volcanic rocks. It is universally admitted, that 

 the cells of volcanic scoria have been produced by 

 aeriform matters disengaged during the process of 

 fusion. Similar cells are found in the trap rocks, a*> 

 I have elsewhere shown; (Chap, xxxix.) and these 

 rocks have also been produced in the same manner. 



Now the cells which, in either of these classes of 

 rock, contain the amygdaloidal minerals, differ in no 

 respect in form and disposition from those which are 

 empty; and if their internal surfaces be examined, it 

 will be found that they are often coated with a similar 

 vitreous varnish. These cavities are not always filled 

 with the minerals which they contain; but present 

 vacuities, in which the crystalline terminations of the 

 minerals are often defined. In the next place, two 

 minerals, or even more, are sometimes found in one 

 cavity; in some cases interfering with each other's 



