LITHOLOGY. 149 
hornblendic varieties are therefore all Dzorite, and the pyroxenic are all 
Diabase. 
When the feldspathic constituent is considered, subdivision is again 
necessary. Often any determination beyond the triclinic character of 
the feldspar is impossible on account of the decomposition, which has so 
nearly destroyed the crystals that only such little particles are left intact 
as suffice for the simplest optical examination; but, on the other hand, 
crystals are often found in such well preserved condition as to allow their 
species to be determined, and to prove that different species exist in dif- 
ferent rocks. In diabase, labradorite appears to be most common and 
most constant; but rocks are found in which anorthite becomes promi- 
nent and easy to determine. In diorite the feldspar is commonly plagi- 
oclase (either labradorite, andesite, or oligoclase), but cases where anor- 
thite is present are not wanting. 
Besides these general sub-divisions, varieties are produced by the 
prominent presence of accessory or variable constituents; and, aside from 
all mineralogical distinctions, there are very marked structural differences 
which sub-divide each species into compact and porphyritic varieties. 
There are, moreover, many varieties formed by degrees of alteration; 
but these are not recognized in classification, and the description of 
these rocks must consist, very essentially, of a history of change, reorgan- 
ization, and decay. It must be borne in mind that such rocks are pecu- 
liarly subject to change, for their mineral compounds are not very stable, 
and their bases are in part readily oxidizable. Expose such rocks there- 
fore to the influences of ages,—to the mechanical movements, change of 
temperature and condition that must have found place in the folded and 
contorted strata of our state,—and this diversity in present condition be- 
comes an interesting feature which might be expected to present itself. 
Besides the rocks mentioned, there is another basic eruptive rock, 
which in its structure and habit is so distinct, that, although composed 
of the same ingredients, and scarcely more than a variety of diabase, it 
may most advantageously be described by itself. This is Gadévo, which 
is considered last. 
DIABASE. 
Diabase is a crystalline granular mixture of augite with a triclinic 
