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 

 inequalit)^ 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 magne- 

 sia, 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 



