SOUTHERN ERUPTIVE CENTER. 237 



not, however, so well exposed, and much less can be said about it. A 

 grand ravine has eaten its way into it from the western side and disclosed 

 at the base propylite and hornblendic andesite in great masses, and exhibit- 

 ing evidence of an early period of great erosion followed by the eruption 

 of augitic andesites and many forms of trachyte, which buried the ancient 

 piles beneath their floods. A few fragmentary exposures of old conglom- 

 erate, consisting of the ruins of the most ancient lavas, are also revealed 

 near the base. Some of these have been so thoroughly metamorphosed 

 that they form almost a homogeneous mass, in which the cement has an 

 aspect closely resembling the fragments it envelops, and is shot through 

 with minute crystals of feldspar and secondary hornblende. When broken, 

 the surface of fracture cuts the pebbles and cement indifferently. The 

 propylites and hornblendic andesites are more profusely charged with horn- 

 blende than those of the northern vent, and the propylites are rather finer 

 in texture. The great mass of rocks now visible in this part of the plateau 

 are of the trachytic series and later in age. They are mostly of the 

 ' argilloid ' varieties, but contain fewer porphyritic crystals of orthoclase 

 than are usually found in such lavas, and are heavily charged with ferritic 

 matter, giving them a dirty brown appearance. Those eruptions which 

 flowed westward commingled with those which emanated from Dog Valley, 

 about 12 to 15 miles westward. Of those which flowed eastward I know 

 but little. I have no doubt that they are well exposed in many of the 

 ravines which descend from the crest of the plateau towards the foot of the 

 Aquarius. I have hastily crossed them once, but have no conception of 

 them sufficiently clear to justify me in attempting to describe them. j\Iy 

 field-notes indicate a broad expanse of trachytic and andesitic rocks inter- 

 bedded with volcanic conglomerate sloping' gently towards the east and 

 appearing to emanate from the above-mentioned source. 



The eruptions from this source did not extend more than 6 or 7 

 miles southward. On the west side of the Sevier Plateau the last that was 

 seen of them was in a deep canon-like ravine, called Sanford Canon, open- 

 ing into Panquitch Valley about 6 miles south of the head of Panquitch 

 Caiion. Here the strictly eruptive part of the plateau ends, and the con- 

 tinuation of it southward is composed of Tertiary beds of the Bitter Creek 



