98 GEOLOGY OF THE HIGH PLATEAUS. 



cialized porphyritic texture, it apparently amounts to this : The ground- 

 mass consists not only of crystals embodied in a base of matter which is 

 not visibly crystalline, but both crystals and base have certain distinctive 

 features ; the crystals of quartz are more perfectly defined in their outlines 

 and possess more distinctly the perfect forms, edges, and angles of their 

 species, the predominant occurrences being the double hexagonal pyramids. 

 The feldspar crystals are also usually distinguished by their perfect forms, 

 especially at the terminations .of the prisms, by their large size and by their 

 many and rare angles. In the volcanics the quartzes are not only fragmental, 

 poorly developed, and of uncertain boundaries, but are often rounded and 

 imperfect at the positions of the edges and angles, while the feldspars are 

 exceedingly irregular and indefinite in shape, not often presenting the well- 

 defined edges and angles distinctive of their species. The base of porphyry 

 is, to a great extent, mysterious and inexplicable. Usually it is (macro- 

 scopically) exceedingly fine-grained, homogeneous, and compact, with no 

 visible trace of crystallization. Under the microscope it presents certain 

 appearances which have puzzled for many years all investigators. With 

 polarized light it exhibits a behavior which is characteristic of crystalliza- 

 tion, and yet no individual crystals can be detected. It is homogeneous in 

 one sense, and yet seems to be minutely granular, as if with greater mag- 

 nifying power and better definition it would resolve into minute crystalline 

 points ; but the latter expectation generally proves a delusion. Not always, 

 however, for sometimes a moderate power resolves the base into a mosaic 

 of crystals, like the groundmass of granite, reproduced upon a microscopic 

 scale. The base of volcanic rocks is usually more or less glassy or fluidal 

 in texture, fidl of microlites, and even when granular is not nearly so much 

 affected by polarized light. 



Many minute characters might be pointed out, but it is needless here. 

 There is no hard and fast line between the porphyritic and volcanic texture, 

 for the latter often simulates the former to a greater or less extent, and 

 even the differences already indicated sometimes vanish or become so 

 poorly pronounced that we fail to apprehend them with confidence. Still, 

 in the long run and in the great mass of cases, we are able to make a 

 distinction, and we find the differences associated witli modes of occur- 



