2 so BULLETIN OF THE 



percentage of silica in all these basalts lies usually between 45 and 55 

 per cent, generally averaging about 49 per cent. 



The unaltered fragmcutal forms of the basalts, as well as of the other 

 volcanic rocks, are called tufas, while I have denominated all the altered 

 and old fragmental forms as porodites, since at present we have no name 

 in the science that covers that class of rocks. The poroditic forms, of 

 course, closely resemble the rocks from which they were derived, and sepa- 

 ration is often impossible in the field, the true character being told only 

 by microscopic examination. To this difficulty of distinction is owing 

 the often repeated assertion that sedinicj:!tnry rocks pass into eruptive 

 ones, — an assertion that has not pi'oved true in the few cases that have 

 come under my obsei'vatiou. It fails, in like manner, under the exami- 

 nation of other investigators, if recent petrographical literature is to be 

 taken as a guide. Classing the various -forms given above as varieties 

 under the species basalt, the scheme is as follows : — 



(Feldspar 1 Basalt ) { Mt-laphyr ] 



Basalt { Leucite L. , ., > Tufa. 



Nephelite jDolente . j)i,^^^ 



! Diabase 

 Gabbro I 

 Peridotite J 



Porodite. 



Serpentine, when derived from the alteration of a peridotite, as is the 

 case with some from California, would be classed under this variety. 



Passing on to the more acidic species, the rocks usually known as 

 andesites come next in order, ranging in silica usually from about 55 

 to 65 per cent, but generally averaging about 61 per cent. We 

 have a gradual passage from basalt, with its globulitic base, rough 

 groundmass, and accompanying minerals, into a rock having a dark to 

 light-brown glass for its base, filled with microlites, and generally 

 somewhat globulitic. Olivine is rarely seen in this ; augite becomes 

 abundant, in well-formed crystals ; plagioclase predominates, but sanidin 

 is more abundant than in the basalt. The color is a blackish-brown or 

 gray, somewhat lighter than in basalt, and the structure of the rock is 

 more porphyritic. The next form, usually classed as andesite, including 

 also some so-called trachytes, has a gray micro-felsitic base, filled with 

 microlites, enclosing similar minerals, and is of like structure but usually 

 of a lighter gray color. Both contain, oftentimes, more or less horn- 

 V)lendo, which plays here the same role that the olivine does in basalt. 

 This hornblende is a mineral of the first class ; its edges are often black- 

 ened, the crystals rounded, broken, blackened on the broken parts, and 

 gnawed into by the molten magma. Sometimes this blackening and 

 destruction extend so far that only little heaps of black grains are left, 

 while similar grains are also scattered through the groundmass. 



