LITHOLOGY. 1 49 



hornblendic varieties are therefore all Diorite, 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 Gabbro, which 

 is considered last. 



Diabase. 

 Diabase is a crystalline granular mixture of augite with a triclinic 



