280 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 fragmental 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 poroditie 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 sedimentary rocks pass into eruptive 
ones, — an assertion that has not proved true in the few cases that have 
come under my observation. 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 | Basalt {Melaphyr 
bast} Leucite | Dolerite : 2 
| Diabase 
Tufa. 
Nephelite 
Gabbro + Porodite. 
Peridotite J 
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 5 
to 65 per cent, but generally averaging about 61 per cent. We 
have a gradual passage from basalt, with its globulitie base, rough 
groundmass, and accompanying minerals, into a rock having u 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- 
blende, which plays here the same róle 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. 
