1870.] MACFARLANE — ON CRYSTALLINE ROCKS. IGo 



centre, and their existence is frequently betrayed by the weather 

 ing of the rock. 



6th. The minerals are here of a much smaller size than in the 

 coarse-grained order, so as to be in most cases difficult of deter- 

 mination. This texture is the same as that often possessed by 

 the matrices of porphyries, and, being destitute of parallel 

 structure, bears the same relation to the coarsely schistose 

 texture. Trap and felsite belong to this order, which may be 

 called i\iQ fine-grained. 



7th. In a small or fine-grained matrix, rounded cavities have 

 been formed, and afterwards partly or wholly filled up with 

 various minerals. On account of the resemblance between the 

 long drawn and flattened shape of these mineral aggregations and 

 almonds, this texture has been called the amijgdaloidal. Trap- 

 pean amygdaloid and the spilite of French lithologists may be 

 cited as examples. 



8th. The next order of texture includes certain fine-grained 

 and globular rocks, characterized by their containing very ap- 

 preciable quantities of water. The globular texture resembles 

 the variolitic, but the concretions, instead of possessing a radiated 

 structure, are composed of concentric layers. Pearlstone is the 

 type of this species of rock, which is intimately connected, 

 geologically, with pitchstone and other impalpable rocks belong- 

 ing to this order. Phonolite and basalt -are examples of the 

 fine-grained members of the order, which, as above-mentioned, 

 are distinguished from the fine-grained order already mentioned 

 by their containing a considerable percentage of water. It may, 

 therefore, be called the fine-grained and hydrated. 



9th. This order may be denominated the trachytic, and, 

 although its rocks have frequently a porphyritic development, 

 they are distinguished from those of that class, in having a rough 

 porous, sometimes even cellular, matrix, and felspar crystals 

 developed in it of a vitreous appearance and full of small fissures. 

 The same rough uneven surface and fracture is developed in 

 those trachytic rocks which contain no largely developed crystals, 

 and even in many of a much more basic composition than what 

 are usually termed trachytes. Pthyolite, andesite and dolerite are 

 examples of this order. 



10th. In this order of texture the porous appearance above 

 referred to is developed to such a degree that a ^coriaceous or 

 cavernous structure results. This structure is peculiar to volcanic 



