ST. JOHx.] VOLCANIC ROCKS — BASALTS. 507 



time. However, this supposed basaltic or lava cap of the watershed 

 perceptibly, though very gently, declinevS to the southwards. As I did 

 not visit the ridge at this point, I am unable to state more fully its mag- 

 nitude in a transverse direction; but it seems very probable that it 

 merely forms a remnant strip, to the presence of which is due the pre- 

 servation of this part of the gi'cat divide. 



Encompassing the head of Bufialo Fork and its mountain tributaries, 

 and constituting the watershed and a largo extent of territory to the 

 ea^st, embracing the sources of the Yellowstone, and, according to Cap- 

 tain Jones, the headwaters of Gray Bull Eiver, and reaching south along 

 tlie continental divide nearly to the parallel of 43^ 30', occurs a vast 

 accumulation of partially sedimented volcanics, the origin of which is 

 almost beyond the comprehension. This mass is chiefly made up of vol- 

 canic breccia^s, with heavy layers of volcanic conglomerate, sands, and 

 mud, the latter of which exhibit unmistakable indications of aqueous 

 deposition, though the former usually shows an aggregation of angular 

 fragments of various volcanic products, trachytes and basalts, held in a 

 tine cement-mati'ix like trachyte-tuflt"; but which in the mass presents ap- 

 l>arently regadarly bedded structure-planes, which give to the exposures 

 in the numerous mountain escarpments the sombre banded appearance 

 by which these deposits may be recognized at great distances. In the 

 region here alluded to they are slightly inclined in various directions, 

 as though they had been subjected to some slight disturbance, at least, 

 by elevatoiy movements located in more or less isolated areas of dis- 

 placement, such as that in which the Buffalo Fork Peak ui^lift resulted 

 which brought to view the crystalline basis rocks and the w^hole series 

 of older sedimentaries of the region. Besides other distiu-bances in these 

 deposits, referred to in the general description of this part of the district, 

 in a preceding chapter, Togwotee Pass appears to occupy the axis of a 

 very shallow depression in the volcanic strata which perceptibly, though 

 gently rise to the northwest and south from the vicinity of this gaj) in 

 the great watershed. 



This deposit may attain in this region a thickness of 2,000 to 3,000 

 feet, possibly even a greater thickness. Entire mountain-ranges have 

 been fashioned from its mass. Including some of the loftiest and most 

 rugged elevations in this region, while the effect of atmospheric ero- 

 sion has produced the most wonderful and sublime mouutain scenery. 

 The observations of Dr. Hayden on the origin of this deposit are so com- 

 prehensive that I beg to quote the graphically expressed results of his 

 owTi earlier investigations in this relation.* "The question arises in the 

 mind. Whence originated this vast deposit of breccia or conglomerate, 

 and what were the physical conditions under which the materials were de- 

 posited ? As to their origin, we must conclude that they were thrown out 

 by volcanoes into the surroimding Avaters much as similar materials are 

 ejecteil from modern volcanoes at the present time. We find, however, 

 that the.se breccias are of immense thickness, sometimes 4,000 to .5,000 

 feet, as at the soiu^ces of the East Fork and in the mountains at the 

 head of the Yellowstone above the lake. Some of the highest mount- 

 ains in the northwest are capped by the^e volcanic breccias, arranged 

 in horizontal strata, and showing most clearly that the agent was water. 

 In almost all cases the stratified breccias are perfectly horizontal from 

 base to summit, thereby indicating the probability that there has been 

 no important movement of the earth's crust since their deposition. We 

 must conclude, then, that at a comparatively modern date the waters 

 so covered these mountain-ranges of the northwest that not even the 



• U. S. Geological Survey of the Territories, 1872, p. 38. 



