48 
AMERICAN GEOLOGY. 
this last property, those rocks which have undoubtedly contri¬ 
buted to the formation of subsequent rocks, have had a great 
influence upon the character of the soil. The most important 
minerals associated with feldspars are hornblende and pyrox¬ 
ene. They are composed of 
Hornblende. 
Pyroxene. 
Silex, 
46-26 
54-08 
Lime, 
13-36 
23-19 
Magnesia, 
19-33 
11-49 
Prot. ox. iron, 
3-43 
10-02 
Ox. manganese, 
0-61 
The fusibility of hornblende and pyroxene is greater than 
feldspar, and as they are both associated with feldspar, they 
probably increase the fusibility of the compound. Hornblende 
and pyroxene rocks, however, decompose more slowly than 
feldspathic, in consequence of the absence of the alkalies. The 
latter rocks, however, contain in this country sulphuret of iron, 
and in consequence of its presence, these rocks undergo greater 
and more rapid changes than they would were they free from 
this substance. The feldspathic rocks, however, contain a 
much smaller proportion of sulphuret of iron; it is not associ¬ 
ated so frequently with it. In the first group feldspar is the 
most important one of its compounds. In this country granites 
are the principal rocks of the group, especially since it seems 
to be proper to include under granites all the mixtures in which 
the three kinds of feldspar are found. In this country we are 
unable to add many of the minor compounds—those, for exam¬ 
ple, which are found in the ancient volcanic districts of Europe, 
as clintstone, porphyry, graystone, diallage rock, &c. 
In certain compounds hornblende or pyroxene predominate, 
though feldspar is always present. 
Epidote should be noticed in this connection. It is no 
uncommon fact to find this mineral where the change in a rock 
is comparatively slight. Chloritic slates, when acted upon but 
feebly by heat, almost always contain epidote. It may be 
massive or crystalline. Its peculiar yellowish green color 
