128 Mr. Dana on the Analogies between the 
We cannot assert that there was ever a period after the ocean 
first covered the earth, in which no land appeared above its sur- 
face. Whether so or not, the lands, at a later period, had 
emerged, and in the shallower waters, but still under some pres- 
sure, porphyry and greenstone were ejected, taking the place 
of the granite or deepsea rock. Sedimentary deposits like those 
of the older fossiliferous rocks may have been in progress. Granite 
was in some parts still thrown out, and not till a very late period 
_ have these eruptions entirely ceased; indeed they may now be 
going on in the ocean’s depths. But on the emerging lands, the 
granitic regions either ceased action entirely, or became porphy- 
ritic or basaltic. ‘These rocks have continued to the present day, 
changing only by becoming more cellular where the eruptions 
were subaerial. : 
Thus we may believe that all the igneous rocks from granite to 
modern lava belong to one series, and were formed by one mode 
of action. 
Partly in elucidation of this subject, and partly:to suggest a 
doubt as to some accredited opinions with regard to the origin of 
certain mountain chains, I will conclude by presenting for con- 
sideration a few hints with regard to the great chain of mountains 
in western America. 
The Andes and Rocky Mountains may be looked upon as ori- 
ginally a grand scene of granitic eruptions. It may have been 
an immense fissure, or much more probably a series of fissures 
ranging in general north and south, over which various granitic 
vents poured forth their granite floods. Granite peaks were thus 
formed, some of which still stand among the highest in northern 
America. 'The Wind River chain, according to the late surveys 
of Lieut. Fremont, is about thirteen thousand feet in eleva- 
tion, and consists of this rock. As the land rose, granites were 
succeeded by eruptions of porphyry, trachyte, greenstone, &c. 
These continued the elevation of the submarine land, by adding 
new streams of molten rock and new beds of porphyry, sand and 
conglomerate ; and together with sandstones and shales of gran- 
ite origin, and limestones of different kinds, they continued 
building up the Andes while still submerged, or with the sum- 
mits only above the waves. Sandstones, shales, salt deposits, 
beds of gypsum, limestones of Silurian and secondary ages, occur 
in the mountains, as on the plains of our continents; and they 
