SEDIMENTARY BEDS— 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.* 



SEDIMENTARY FORMATIONS OF THE MARKAGUNT. 



Around the western and southern borders of the Markdgunt extends a 

 broad belt of sedimentary formations almost wholly unencumbered witli 

 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 sedi- 

 mentary strata from the Eocene to the base of the Trias inclusive. 



The most recent deposits are those local accumulations first encoun- 

 tered 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 indittcrenco to minor topographical features 

 by sjiying 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 tho 

 canon itself, though tho stream of lava often runs into tho canon. So numerous, indeed, are the in- 

 stances of cones upon tho verge of a clift' of erosion or canon-wall, that I was at one time led to suspect 

 that it was a favorite locality. This is very conspicuous in the large basaltic field near tho Grand Canon 

 in the vicinity of Mount Trumbull, where 10 large cones stand upon the very brink of the great abyss 

 and have sent their lavas down into it. Away from tho canon a considerable number of craters aro 

 seen upon tho various cliffs near the Hurricane Ledge, and far to the northeastward half a dozen aro 

 found upon thi; 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., occupying such positions. Whether this is accidental it is diffi- 

 cult to say, but when it is remembered that they do not occur at the bases of such clifl's, nor in tho 

 c.inons (so far as I have observed), tho 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 njion tho reasons for this apparent preference of locality. 



