SUBiERIAL PYROPLASTIC ROCKS. 
113 
The ranges of trap in the far west begin to make their 
appearance just beyond the verge of the crests which divide 
the waters of the Missouri from those of the Columbia and 
Colorado. The Rev. Mr. Parker many years since described 
the basalts and greenstones upon the vrestern slope of the 
Rocky mountains. These eruptive rocks appear not far from 
the head waters of the Columbia and Colorado upon the Pacific 
slope. A large proportion of Oregon, indeed, is basaltic. At 
the head waters of the Salmon river the gneissoid granite 
begins to give place to the pyroplastic rocks—greenstone, por¬ 
phyry, amygdaloid, and basalt—the latter of which walls up 
the Columbia at many points, as at the Cascades, Willamette 
falls, Grande Coule, Walla Walla, the Dalles, Cape Horn, 
Smoke river, &c. The boundaries of this great region of trap 
remain, however, undetermined. It is the largest and most 
interesting geological field of this formation in the United States. 
SUBiERIAL PYROPLASTIC ROCKS. 
§ 83. These rocks are erupted from craters, which in their 
perfect forms are perforated cones formed over a tubular aper¬ 
ture leading to the subterranean incandescent and melted mat¬ 
ter beneath. The beginning of a cone is a fissure, simple or 
stellated, which in giving vent to the melted matter is modified 
in form and condition. The line of fissure will be filled by 
erupted matter, while the orifice is rounded by the exit of incan¬ 
descent lava. The formation of cones has often been observed. 
Their shape and structure are very uniform. The material is 
derived from rocks which are melted and partially altered at 
unknown depths, and being forced upward, escape through the 
vent, either at the apex or at one side of the cone. A cone 
will consist in part of consolidated erupted matter collected 
around the vent. If all the substances formed beneath consisted 
of fused or liquid rock, flowing out at one side, the symmetrical 
cone would not be formed at all: but as ashes and cinders are 
ejected, they fall around it, and in the course of a few hours a 
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