1881 .] 
283 
[Crosby. 
If the particles are macroscopic, they may be divided again 
according to form; and undoubtedly the most important distinc- 
tion to be noted here is that between crystals and crystalline par- 
ticles on the one hand, and irregular or water-worn fragments on 
the other. And thus, as shown in the table, we arrive at a defi- 
nition of two of the most important textures — the fragmental 
and the crystalline. There are numerous varieties of each of 
these based upon peculiarities in the forms or sizes of the parti- 
cles ; thus among the fragmental textures we have the arenaceous , 
conglomerate , breccia , etc. These are the granular textures ; but 
the term granular , being applicable to both the fragmental and 
crystalline textures, is not a good name for either. To be spe- 
cific, we must say granular fragmental or granular crystalline. 
If the particles are microscopic, we cannot, of course (without 
using the microscope), divide them according to form ; but, para- 
doxical as it may seem, they can be divided according to size. 
We can distinguish those rocks in which the particles are 
merely minute, becoming visible under the microscope, i. e ., those 
rocks in which the texture is simply finely fragmental or finely 
crystalline, from those in which the constituent particles (acci- 
dental impurities and inclosures aside) are, for aught that we can 
determine, of molecular smallness, not being resolved by the 
highest powers of the microscope. The rocks of the first class, 
of which common clay-slate is a good example, are called com- 
pact ; and those of the second class, presenting, as in obsidian, a 
perfectly continuous surface and glassy lustre, are termed vit- 
reous. 
The four textures which we have now defined, and which, it 
will be observed, are determined entirely by the forms and sizes 
of the constituent particles, I have called the primary textures ; 
because every rock must possess one of them. We cannot con- 
ceive of a rock which is neither fragmental, crystalline, compact 
nor vitreous ; but it is not rare to find two of these primary tex- 
tures, as the vitreous and crystalline, or compact and crystalline, 
combined in the same rock. Obsidian with interspersed crystals of 
sanidine is common, and is a sufficient refutation of the notion 
advanced by some writers that a rock cannot possess two textures 
at once. 
