Crystalline Structures of Rocks. 73 



The fibrous structure is the last which can strictly be enume- 

 rated among; the concretionary modifications; and it seems to unite 

 them with those that are properly of a crystalline nature. It is 

 known to occur in the carbonates of lime, as in the satin spar, and 

 in the limestones of Egg. In the former, it is more decidedly crys- 

 talline than in the latter, resembling the corresponding arrange- 

 ment so frequent in gypsum. It is also not very uncommon in the 

 argillaceous schists ; in which, as these are not susceptible of the 

 crystalline arrangement, it must necessarily be referred to the con- 

 cretionary structure. Its cause involves exactly the same difficulties 

 as those which attend the explanation of the schistose structure. 

 It is only necessary to observe of many other fibrous arrangements 

 seen in rocks, including that which has been called bladed, that 

 they are purely crystalline ; their peculiar aspect being produced 

 by the lengthened forms and parallel arrangements of the crystals. 

 Of the scaly structure, it is unnecessary to say more than 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 the flat parts or scales, it requires no notice in this 

 place. 



Of the Porphyrific, Oranular, and Amygdaloidal Structures. 



The structure called porphyritic, is purely crystalline, and is that 

 which confers the peculiar character on the porphyries. It is by no 

 means, however, deficient in interest ; as it is only known in those 

 rocks which appear to have derived 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 exist- 

 ence of the porphyritic structure in volcanic rocks affords every 

 proof of the nature of its origin that can be desired. 



The granular structures that belong to the sandstones and con- 

 glomerates, being purely mechanical, need not be noticed ; but 



