& 
Modern Igneous Rocks and the Primary Formations. 129 
form in some parts, plains of immense extent, underlaid by hori- 
zontal strata. ‘The depositions then took place in the same man- 
ner as on the low lands of other regions. The reason why a 
submarine mountain chain was formed, and not a flat continent, 
is the very simple one that the sources of all the material of the 
mountains lay nearly in one and the same line; for igneous erup- 
tions threw out the material and piled up the mountain. ‘There 
are two peculiarities in the structure of the mountain regions— 
the first is the great predominance of sandstones of porphyry ori- 
gin, and which are not common in our plains; but this should be 
expected from the nature of the source where the material of the 
mountains was derived. 'The second is the broken character of 
the mountain heights. 'This is much less than is commonly be- 
lieved, fora considerable portion of the range is covered with ele- 
vated plains. Yet there are gorges and valleys of great depth, and 
heights lofty and abrupt ; yet nothing more than should be ex- 
pected in a region of the most extensive igneous action in the 
world. There are variously displaced and tilted strata, attributa- 
ble to local convulsions in the range. 
In this manner we may suppose the Andes to have been mostly 
formed below the sea. Next, by gradual expansion below, or some 
other cause, an elevation commenced in the latter part of the tertia- 
ry era, if we may rely on the occurrence of tertiary rocks on the 
summit. Igneous eruption continued, but diminished with the ele- 
vation. ‘The mountains slowly emerged, and continued raising 
their heads aloft till the continent at its foot had also appeared. 
During this elevation it was subjected to a tearing ocean, and 
thus its shattered sides were still farther gorged out. 'The eleva- 
tion may not have been equal over the entire continent, and in all 
probability the eastern side participated but little comparatively 
in the motion, and was early dry land. 
These views with regard to this chain of mountains are thrown 
out merely as general hints, and as thoughts to be hereafter tested 
by observation, rather than truths worthy of immediate confi- 
dence. 
Vol. xiv, No. ].—April-June, 1843. 17 
