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Show SEDIMENTAEY BEDSâ€"TUFAS. 203 On the other hand, an equally striking fact is the apparent independence of basaltic eruptions of the minor or local inequalities of a country. They have broken out, with seeming- indifference, upon hill-tops and slopes, in valley bottoms, upon the brinks of great cliffs of erosion, upon buttes, and upon broad mesas. The only localities where I have not seen them are in canons and at the bases of cliffs of erosion.* SEDIMENTAEY FORMATIONS OF THE MARKAGUNT. Around the western and southern borders of the Markagunt extends a broad belt of sedimentary formations almost wholly unencumbered with volcanic emanations. The volcanic cap ends always abruptly upon the highest part of the plateau several miles from the plateau limits, and usually presents to the westward a line of cliffs looking down into the great valleys and amphitheaters where the ravines and canons of the sedimentary belt begin. The destroying agents have wrought terrible havoc in the strata, cutting chasms which have laid bare in grand sections the series of sedimentary strata from the Eocene to the base of the Trias inclusive. The most recent deposits are those local accumulations first encountered in Bear Valley, consisting of the sands and marls derived from the decay of volcanic rocks. We seldom miss them from their proper place at the base of the volcanic cap, and they attain considerable thickness (200 to 350 feet) in numerous exposures along the western margin of the trachyte. From what rocks they were derived it is impossible to say; no lavas older than themselves have been detected. They rest everywhere upon the Eocene limestones, frequently shading downwards into sandstones undis- * Perhaps I ought to qualify this assertion of seeming indifference to minor topographical features by saying that basaltic vents occur very often upon the brink of cliffs of erosion, and never (within my own observation) at the base of one; often upon the top of the wall of a canon and never within the canon itself, though the stream of lava often runs into the canon. So numerous, indeed, are the instances of cones upon the verge of a cliff of erosion or canon-wall, that I was at one time led to suspect that it was a favorite locality. This is ve'ry conspicuous in the large basaltic field near the Grand Canon in the vicinity of Mount Trumbull, where 10 large cones stand ivpon the very brink of the great abyss and have sent their lavas down into it. Away from the canon a considerable number of craters are seen upon the various cliffs near the Hurricane Ledge, and far to the northeastward half a dozen are found upon the crests of the White Cliffs. Out of rather more than 300 basaltic cones of this region, I have noted 33, or nearly 11 per cent., ocenpying such positions. Whether this is accidental it is difficult to say, but when it is remembered that they do not occur at the bases of such cliffs, nor in the canons (so far as I have observed), the fact is certainly a remarkable one. In our present ignorance concerning the nature of the forces and chain of causation which lead up to and precipitate volcanic phenomena, it would be vain to speculate upon the reasons for this apparent pi'eference of locality. |