a 
a 
i eset e 
Origin of the Constitution of Igneous Rocks. 351 
portions, the rock is compact. Whatever augite may be present, 
instead of appearing as augite, will take the form of hornblende, 
a mineral which requires slow cooling, and differs from augite in 
the crystalline form which it thus receives. In corroboration of 
this statement, hornblende is common in trachytes and such feld- 
spathic rocks. The same remarks apply to mica: and other min- 
erals also may form according to the elements present. Chryso- 
lite is not met with: it occurs only where there is a more rapid 
sen of cooling, as in the formation of ordinary basaltic rocks or 
_ Farther we observe that with a still more gradual rate of cool- 
ing, the whole feldspathic rock becomes crystalline in’ texture 
like a granite or syenite, and it is well known that granite-like or 
syenitic rocks or peaks occur in some volcanic regions, whose 
iterior has been laid open by denudation. Many minerals too 
might crystallize under these circumstances, which with more 
tapid cooling would not be distinguishable. 
The boiling process in a large volcano, therefore, in connection 
With the circumstances of temperature, rate of cooling, the fusi- 
bilities of different minerals, and the other causes alluded to, will 
4ccount for the various features, positions agd relations of igne- 
ous rocks, and for many facts relating to the distribution of igne- 
ous minerals.* We may hence reasonably infer that granite and 
granite minerals may form under the same circumstances, if the 
ents are present in the material in fusion ; for the syenites al- 
luded to are closely allied rocks in texture and character. Ata 
7 former Meeting of this Association, it was suggested by me that 
Some regions of granite peaks may have been centres of ancient 
'§heous action ; and their being surrounded or bordered by horn- 
blendic rocks, seems to point to some actual analogy with the tra- 
chytic centres and basaltic circumference of mountains admitted 
tohave been volcanic. . 
_ The opinion that the nature of the resulting rock is directly 
Connected with the nature of the rock which had entered into fu- 
; Sion, cannot be maintained if the above views be true. On the 
Contrary, it appears that while the result may thus be varied, the 
; : tennant ie fir Sect aw omaeeer see 
"The general causes coheed to, act under the guiding laws of crystallogeny, 
Which laws regulate the particular positions of minerals according to the principles 
emplified in segregations or radiated crystallizations, and the laminated or clea- 
vable structure of j eile: 
