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Show 62 GEOLOGY OF THE HIGH PLATEAUS. a very few centuries. The cones are perfect, the lava is not faded by time, and even the spongy, inflated scum of the surface is still black as coal or faintly tinged by atmospheric reagents. That the basaltic period was a long one is further manifest by the fact that on the southwestern flank of the Tushar is a conglomerate composed wholly, or nearly so, of basaltic materials. These were derived from the degradation of the massive basalts, which have overflowed that part of the range, and they are well stratified after the peculiar manner of sub-aerial conglomerates. The basalts, in choosing localities for eruption, show here a tendency to abandon those parts of the district which had been the seats of the grander outbreaks of earlier periods and to find new and independent localities for their extravasation. It is not always so, however, for the greatest basaltic floods outpoured hard by one of the most important centers of tra-chytic eruption. But, on the whole, their situation relative to the older masses is peripheral. In the Markagunt the greater part of the basalts lie upon the sedimentary beds. In addition to this, we find many lone vents, or a small cluster of them, standing far away from the central fields of more ancient lavas. A large number of basaltic streams have emanated from the very walls themselves. In truth, no one can fail to be struck with a peculiar habit which they manifest of seeking strange places from which to break out. Very many cones are perched upon the brinks of the terraced cliffs or cafion walls. In the western wall of the Paunsaffunt the lava has broken out from the very face of the wall itself. The least common place for a basaltic crater is at the base of a cliff. In a great majority of cases the vents stand near the faults, but the curious part of it is that they break forth almost always upon the lifted and very rarely upon the thrown side of the fault. All of the basalts are of the feldspathic varieties, none of the nephelin and leucite bearing varieties having been met with. THE ORDER OF SUCCESSION IN THE ERUPTIVE ROCKS. The views of F. Baron Richthofen on the succession of eruptions* have received from American geologists profound attention. Probably no Memoir presented to the California Academy of Sciences * A Natural System of Volcanic Rocks, by F. Baron Richthofen, May 6, 1867. |