352 J.D. Dana on the Volcanoes of the Moon. 
mode of distributing minerals in a volcanic focus by the boiling 
sess, may produce from the same material, rocks of a predom- 
inant feldspathic character in one place, and rocks of a hornblen- 
dic or augitic character in other places. Simple feldspathic gran- 
ites may be fused and ejected as feldspathic rocks, like those of 
porphyry dikes. But it is an interesting fact, that the rock of 
most dikes is of the augitie (or hornblendic ) kind, like the dikes 
of voleanoes that rise from sources in which the giptrio 
process could not have been operating.* 
We also arrive at the important conclusion, that rocks perfectly 
compact in texture may be of subaérial origin, as we have: pres- 
sure from the fluid lavas themselves in the volcanic focus.. 
_ Another deduction proceeds from the facts stated ;—that the 
same igneous rocks may occur of all ages, provided the atmo- 
sphere or waters of the earth were not too warm for the more 
rapid rate of cooling required. for uncrystalline rocks. Seorias, 
basalt, trap, porphyry, syenite, granite, have no relations to one 
pbosk rather than another, beyond what may depend on the cir- 
cumstance just mentioned. Whenever therefore in the history 
of the world, the variations in heat, pressure, and rate of cooling; 
now possible, may have taken place, similar rocks to those of the 
present day may have been in progress :—and as far as the varia- 
tions of former times, in these respects, may now take place, for- 
mer rocks and minerals may still be in progress. In this state- 
ment it is implied that the necessary elements are pores in the 
fused material. 
IIL. Origin of Continents.—The moon gives us hints on anoth- 
er topic of great interest, relating to the distribution of land and 
water on our globe. We have mentioned that there is a large 
area covering nearly one third of the hemisphere facing the earth, 
which is mostly free from volcanoes, while on other parts the 
craters are closely crowded together. We may therefore reason 
* Mr. Darwin has accounted for the distribution of feldspathic and aug tic rocks 
in volcanoes, on the ground of their different specific gravity, But with this cause 
heavier augitic material, which is not the case. He also argues that the feldspar 
would rise in the fluid as crystals, and so the au gite sink. But we know in the first 
place, that crystals do not appear till incipient ectidibiontions and if the augite and 
feldspar were both in distinct crystals, where would be hi fusion? Again, the 
elds are , except with a very slow rate of cooling; wert 
then can the existence of erystals be assumed ? 
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