CRYSTALLINE STRUCTURES OF ROCKS. 183 



peculiar aspect being produced by the lengthened 

 forms and parallel arrangements of the crystals. 



Of the scaly structure, it is sufficient to say that it 

 is one of those which, when it occurs in rocks of a 

 crystalline character, must be considered as among the 

 first in the order of crystalline arrangements. As a 

 consequence of the mechanical deposition of flat par- 

 ticles or scales, it requires no notice in this place. 



Of the porphyritic, granular, and amijgdaloidal 

 Structures. 



The structure called porphyritic is purely crystal- 

 line, and is that which confers the peculiar character 

 on the porphyries. It is by no means however de- 

 ficient in interest ; as it is known only in those rocks 

 which, from many circumstances, are proved to derive 

 their origin, from fusion. When indeed we consider, 

 that in this case, a single crystal of a perfect form is 

 surrounded by an uncrystallized mass, it. offers in 

 itself a proof of the species of fluidity under which the 

 whole must have been consolidated. No imagination 

 can assign an expedient for producing this effect from 

 a watery solution; while the existence of the porphy- 

 ritic structure in volcanic rocks, affords every proof of 

 the nature of its origin which can be desired. 



The granular structures which belong to the sand- 

 stones and conglomerates, being purely mechanical, 

 need not be noticed ; but that of granite and the ana- 

 logous rocks, being of a crystalline nature, is here 

 deserving of regard. It has been maintained that this 

 structure has been the produce of watery solution, by 

 those who still chuse to consider granite of aqueous 

 origin. The argument, as far as its texture or struc- 

 ture is concerned, belongs properly to this place. 



