| OCR Text |
Show 198 GEOLOGY OF THE HIGH PLATEAUS. cluster of rolling hills, rarely exceeding 300 feet in altitude above the platform on which they stand, covered with soil mingled with decaj^ed vesicular cinders. Their true nature is disclosed by the scoriaceous character of the fragments which constitute the greater portion of their mass. It will be remembered that basaltic craters, when well preserved, are rather symmetrical truncated cones, with conical or funnel-shaped depressions at the summit, and the entire mass is composed of vesicular fragments blown out by the escaping steam and gases and falling with approximate uniformity around the orifice. The spongy character of these fragments renders them an easy prey to the chemical forces of the atmosphere, and they are readily decomposed. After thousands of years of weathering these cones are literally dissolved, losing their lime, iron, and alkali, while the alumina and silica remain, and the cone gradually loses its form and is reduced to a shapeless heap of soil with commingled cinders in every stage of decay. Around the bases of these ancient cones we find half-revealed sheets of basaltic lava. Any eruption may be followed by the building of a cinder-cone, and most basaltic outbreaks are so supplemented (at least in this district) ; but it is not always so. A considerable number of the basaltic sheets have been disgorged where no trace of a cone remains, and some of these are so recent that the last thousand years may have witnessed the catastrophes.* It is notable that the most extensive outpours are most frequently without them. Among the basalts of the locality of which we are speaking are many cinder-cones in an advanced stage of decay. The floods of basalt which have emanated from them lie in many sheets, none of which individually present great thickness, but by superposition have built up this part of the plateau from 500 to 800 feet above the normal platform. They are for the most part concealed by their own ruins, but numerous ravines have been cut into them, showing in many places their edges and giving a general idea of their mass and distribution. They rest upon older trachytes and occasionally andesites which had been scored by ravines before the basaltic outbreaks, and in a number of places the uneven surfaces of contact are clearly revealed. * I am speaking in general terms of the basalts. Those of the locality just spoken of are all probably older than the Quaternary. |
