OCR Text |
Show 80 GEOLOGY OF THE HIGH PLATEAUS. and under the microscope it shows a groundmass of the same texture and composition. Crystals are frequently seen lying partly in the original pebble, partly in the original matrix, and the surfaces of fracture betray no inequarhy of hardness or cleavage, but cut through the pebbles and matrix indifferently. Microscopic examination discloses a groundmass, differing in no very important respect from such as are displayed by many eruptive rocks. The base, however, has, in all the instances which I have examined, that felsitic aspect which is characteristic of porphyritic rocks, neither glassy nor strictly microcrystalline, but exhibiting that aggregate polarization which is not yet satisfactorily explained. There is an entire absence of glass or fusion products in the groundmass. Free quartz is often found even in those varieties which consist largely of plagioclase and hornblende or augite. The fragmental character of the matrix has disappeared; not a trace of the original clastic condition can be detected, unless it is to be found in some of the quartzes and feldspars. I see nothing at all incredible in the idea of metamorphism producing rocks so closely resembling some eruptive rocks that they cannot be petro-graphically distinguished from them. It seems rather that we ought to anticipate just such a result from the alteration and consolidation of pyro-clastic strata. The materials which compose them consisted originally of disintegrated feldspar, pyroxene, and the matter which constitutes the amorphous base of all eruptive rocks. In general they are silicates of alumina, alkali, lime, magnesia, and iron, from which, no doubt, portions of the soda, lime, and silica, and to a less extent the iron, potash, and magnesia, originally forming the massive rocks from .which they came, have been abstracted by atmospheric decomposition. They still retain portions of all these constituents, and only require the presence of conditions favorable to reaction in order to generate feldspar, mica, hornblende, and, perhaps, fresh quartz. Ordinarily we should anticipate that only small quantities of soda and lime would be present, and inasmuch as these bases are necessary to the formation of feldspar (plagioclase), only a partial crystallization would result. There would be left a considerable quantity of aluminous silicate, with some magnesia, which might form mica or aluminous hornblende, though the greater portion of it would ordinarily remain as an amorphous felsite |