164 THE CANADIAN NATURALIST. [June 



rocks, which also afford examples of purely vitreous texture, in 

 which no "grain " nor any mineralogical constituents are observ- 

 ablCj but an impalpable glassy appearance predominates. This 

 order may be called the lava texture, and lava pumice-stone, and 

 obsidian, mentioned as examples of it. 



It is not to be supposed that these varieties of texture are at 

 all sharply separated from each other. On the contrary, rocks 

 the most varied in their structure are found to be connected with 

 each other by insensible gradations. Thus, vitreous rocks are 

 gradually found to assume an impalpable and then stony 

 character. Then again, they frequently become porous and 

 cellular, and graduate into scoriaceous lavas. Rocks of the latter 

 order have very often well-defined minerals developed in them, 

 and when also the cellular texture becomes more subdued, 

 trachytic rocks result. These, when they gradually become more 

 compact or their felspars gradually lose their vitreous and fissured 

 appearance, become indistinguishable from felsites and porphyries. 

 Further, when the matrices of the last-mentioned rocks gradually 

 become coarser grained and their crystals reduced in size, they 

 pass into thoroughly granular rocks. When, on the contrary, 

 the well-developed crystals of porphyries gradually disappear, 

 fine-grained rocks are the product. Nothing is more common 

 than to find the latter gradually assuming a slaty structure or 

 gradually becoming coarser in the grain, and so giving rise to 

 schistose or granular rocks. And nothing is more common than 

 to find the constituents of granular rocks, little by little, arranging 

 themselves in a given direction, and so producing coarsely schistose 

 structure. 



But with all the frequency of gradation between original rocks 

 of various textures, it is to be remarked that those which differ 

 widely from each other in structure, do not exhibit sudden transi- 

 tions the one into the other. Cavernous and coarsely granular 

 rocks are never found to constitute part of one and the same 

 mass, or to pass into each other, without gradually assuming the 

 character of intermediate impalpable and fine-grained rocks. Nor 

 is it ever the case that coarsely schistose rocks become trachytes 

 all at once. A certain consistency or method is recognisable in 

 all these transitions, and it is only those orders which are more 

 nearly related to each other as regards texture, or arc more 

 intimately associated, geologically, that graduate into each other 

 ^n the manner above described. In the description of tliQ various 



