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 Mark&gunt 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 brinies of the ter- 

 raced cliffs or canon walls. In the western wall of the Paunsagunt 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 



* A Natural System of Volcanic Rocks. Memoir presented to the California Academy of Sciences 

 by F. Barou Richthofen, May 6, 1S67. 



