| OCR Text |
Show NORTHERN FOCUS OF ERUPTIONS. 233 say. To the westward they are cut off in the great wall which faces Sevier Valley with an altitude of nearly 6,000 feet above the river. To the east they are likewise cut off by the oblique valley, though they reappear at lower altitudes on the other side, and are instantly lost again under soil and waste, but evidently descend into Grass Valley, and may commingle with the equally grand floods emanating from the Fish Lake Plateau to the eastward. But south, and north they are displayed in immense volume. Those which flowed north and northeast are spread out in the vicinity of Salina Canon and one great coulee stretched beyond the canon, which now cuts off a portion of it, leaving it as an outlier. Large portions of these old lavas have been swept away. The mauvaises terres south of Salina village were once covered with it. Standing prominent among these bad lands is a conical butte-like mountain of singularly perfect form. It is a remnant left by circumdenudation, and upon its summit is a "tip" or cap about 250 feet thick, consisting of this same lava reposing upon the sedimentary strata, out of which the peak has been carved in cameo. This mountain is called the Black Cap. The augitic trachyte,* of which its summit apparently forms a remnant, is the same as that which extends across the Salina Canon. This flow reached a distance of 30 miles from its source. South of the canon and nearer the source sheets of argil-loid trachyte rest upon the augitic and hornblendic, and heavy beds of conglomerate derived from the ruins of both kinds of rock are interspersed. To the northeastward, extending as far as 25 miles, similar aggregates of massive superposed coulees are displayed, having a thickness of nearly a thousand feet and increasing in bulk as we approach the Sevier Plateau. The hornblendic trachytes are in the larger proportion, but the lighter gray trachytes, and especially the 'argilloid' varieties, are almost as voluminous. They are much degraded by erosion, and several fine canons have been cut, ramifying into broader ravines, with big rough swelling hills between them. * This rock is a conspicuous one. It has many crystals o( sanidin, but the less conspicuous plagio-clase is very abundant. The line is difficult to drawâ€"perhaps impossibleâ€"between some andesites and augitic trachytes. The texture is sometimes the only basis of a distinction, and this should be used with great caution, and never without reservations. Still the textures of the two groups arc usually distinct and characteristic, and the rock assumes in most cases the one aspect or the other even when the mineralogical constitution is doubtful. In the very few cases where there is no means of forming a decided distinction it would seem as if the old term " trachydolerite" might be useful. It has the advantage at least of being non-committal. |
