162 THE CANADIAN NATURALIST. [June 



a crystalline rock. These instances have been given in order to 

 show that, in studying the varying textures of original rocks, it is 

 "well to bear in mind that such textures are, in all likelihood, the 

 result of the influence of the physical conditions under -which 

 their respective rocks solidified, and of the temperature and 

 plasticity of the mass from which they were produced. 



The followin": modifications in the texture of orisrinal rocks 

 may here be distinsuished : — 



1st. The constituent minerals are of a comparatively large size, 

 ranging from several inches to one eighth of an inch in diameter, 

 generally large enough to be easily tested as to hardness, cleavage, 

 and other physical characters. The mode of their arrangement 

 is altogether irregular, and, although the individual minerals may 

 sometimes have a greater length than thickness, no parallelism of 

 their larger axes can be noticed. Granite, syenite, and diorite 

 are examples of this order of texture, which may be called the 

 coarse and small grained. 



2nd. The constituent minerals are of a size varying from the 

 smallest individuals to those of an inch in diameter. One or 

 more of them have their longest axes arranged in the same 

 direction and parallel with each other, there being thus developed 

 a fibrous or laminated texture. This may be called the schistose 

 order, to which gneiss and hornblende schist belong. 



3rd. The constituent minerals are finer grained than in the 

 preceding order, and more difficult of determination. A similar 

 parallel structure, however, is visible, which occasions an easie 

 fracture of the rock along a particular plane, or what is called a 

 slaty cleavage. Common roofing slate may be regarded as the 

 type of this slaty order of texture. 



4th. The next order of texture to be distinguished is the 

 porphjritic. Large individuals, or crystals of one or several 

 minerals, are enclosed in a fine-grained or impalpable matrix. 

 Augitic, syenitic and felsitic porphyry are examples of this order 

 of texture, the rocks of which are distinguished from each other 

 as well by variations in the nature of their matrices as in the 

 compositions of the crystals developed in it. 



5th. The next order may be called the varioUtic, and regarded 

 as incipient porphyritic texture. In a fine-grained matrix, small 

 rounded concretions are developed, without, however, being 

 sharply separated from it. These concretions sometimes possess 

 a fibrous structure in the interior, the fibres radiating from the 



