46 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



they belong to the same series of conglomerates that occur about Tower 

 Falls, and form the walls of the canon above Tower Creek. They are 

 apparently continuous with the formations that constitute the bulk of 

 Amethyst Mountain, and which by the evidence of their flora are Miocene 

 or Pliocene. The interesting feature is this: that a number of beds or 

 sheets of basalt are intercolated with the conglomerates in the wall of 

 the canon. This basalt is beautifully columnar, as every visitor to the 

 region knows, and the outcropping edges of the sheets are very uniform 

 in thickness and quite persistent horizontally. It is safe to say that 

 they are not intrusions but contemporaneous flows, and therefore, since 

 they are interbedded with the conglomerates, that they are older than 

 the rliyolites of this region, as the latter uniformly overly the conglom- 

 erates. 



On the opposite side of the caiion the butte o is capped with a mass 

 of basalt underlaid by rhyolite, beneath which are the conglomerates. 

 Table rock at Tower Falls is also formed of basalt. The bed to which 

 it belongs is quite massive, but so far as I have seen not columnar. 

 In the Avails of Tower Creek, above the trail, this bed of basalt curves 

 down and apparently extends beneath the bed of the creek. Under 

 the basalt are greenish sandstones and conglomerates. That the 

 strata upon which the basalts and rhyolites of the south side rest are 

 Tertiary I have no doubt. The beds in which the columnar basalts of 

 the opposite side lie are apparently the same, but may possibly be more 

 recent. An examination of the conglomerates ought to determine this 

 question, since, if they are younger than the other strata of the region 

 and subsequent to the rhyolites and trachytes, they will contain frag- 

 ments of the rhj'olitic rocks which abound in the locality from which their 

 materials must be derived. I am strongly of the opinion, however, that 

 none of these rocks will be found in this formation, and hence expect 

 that time will demonstrate that all these valley conglomerates are older 

 at least than any of the rhyolites. 



I present two'sections, A and B, Plate XXVI, which will show the re- 

 lations of the various formations of this part of the valley. 



With the lower end of the valley I have only a limited acquaintance. 

 West of Elk Creek a steep, irregular slope lejids up to a high promon- 

 tory — an outstanding spur of the Washburn Mountains — that overlooks 

 the third canon of the Yellowstone, opposite the mouth of Hell Eoaring 

 Greek. In ascending this eminence by way of the trail from Elk Creek 

 vre i^ass up very steep slopes to a gap or saddle just west of the 

 highest point of the promontory, and find at the base some 500 feet of 

 the brecciated trachytes. These rocks seem to outcrop all along the 

 Elk Creek base of the promontory. They are identical with the brec- 

 ciated trachytes of the valley. The main wall of the promontory is com- 

 I)Osed of Tertiary strata, sandstones, and conglomerates, with possibly 

 some Post-Tertiary conglomerates above. 



In 1872, Dr. A. C. Peale obtained a number of species of Tertiary 

 leaves from the sandstones near the trail on the Elk Creek side of the 

 promontory. Toward the base, on this side, the sandstones are fine- 

 grained and gi'ayish and greenish in color. They alternate with com- 

 pact conglomerates, and are pretty evenly bedded. The materials which 

 form the conglomerates are only moderately coarse, and are chiefly, if 

 not altogether, composed of basaltic or andesitic materials. The mass 

 is frequently so compacted that it breaks with an even fracture, the sec- 

 tions of the constituent fragment appearing on the surface. These frag- 

 ments are never well rounded, yet do not seem to be sharply angular. 

 Higher up and in the sides of the gorge, through which the trail passes 



