26 GEOLOGICAL SUEVEY OF THE TERRITOEIES. " 



somber, apparently basaltic rocks, whicb, except for tbeir structure and 

 color, I would regard as Cretaceous or Tertiary. The whole series is 

 arranged in beds of marl, with more or less compact layers of harder rock, 

 which project out the. same as in those formations. In these upper beds 

 I found fragments of wood, and in the uppermost beds were fragments of 

 leaves, which I cannot but regard as of Tertiary age, and that the whole 

 series of beds have been greatly affected by heat so thut the lowest beds 

 have been entirely changed. Passing up the mountain we found our- 

 selves in a synclinal basin, with the strata dipping at a low angle, those 

 at the southeast at an angle varying from 30° to 60°', apparently com- 

 I)rising the difterent formations from Tertiary to Carboniferous. The 

 rocks do not show so many signs of heat as heretofore. Our course has 

 been directly north, and mostly through Carboniferous rocks, dipping 

 about southeast at an angle of 20°. There are, in the caiion that we passed 

 through, at least 1,000 feet of limestone exposed, and as we leave the • 

 canon northward we find 200 to 300 feet of red marly limestones, much 

 like the red deposits we have before met with, only harder. These rocks 

 are peculiar, differing from any before seen. They pass from a red loose 

 slate down into a compact clay-slate, gradually varying from, a deep red 

 to black thin slates, becoming more and more compact as we descend, 

 until they appear to be a melted rock, and the joints are so close that 

 they separate the whole mass into small fragments. The rock does not 

 effervesce at all with sulphuric acid, but is of a very compact texture. 

 In regard to the age of these beds I can form no exact idea, no fossils 

 having been detected, though frequent sun-cracks are seen upon the 

 surface of the slates." 



The following paragraph, describing one of the four passes near 

 Henry's Lake, is takeu from the report of Colonel W. F. Raynolds of 

 bis explorations in 1800, page 98 : 



"The pass is only four miles from, and 200 feet above, the lake, and 

 so level that it is dilficult to locate the exact point at which the waters 

 divide. It is about a mile in width, with the sides sloping gently to the 

 center. The barometer stood at 23.05 inches, indicating a height of 

 0,350 feet above the sea-level, or 1,500 feet lower than the summit of the 

 South Pass. The approaches upon either side are remarkable, being of 

 about a uniform ascent of 50 feet to the mile, and thus affording un- 

 equaled facilities for either wagon-road or railroad purposes. I named 

 it Low Pass, and deem it to be one of the most remarkable and 

 important features of the topography of the Eocky Mountains." 



This beautiful pass has been so carefully described by Colonel Eey- 

 uolds that I gladly record its name on an official map asKeynolds'sPass; 

 the name Low Pass, given it by Colonel Eeynolds, not being sufficiently 

 distinctive for a geographical name. 



CHAPTEE 11. 

 GALLATIN VALLEY— YELLOWSTONE VALLEY. 



In my annual report for 1871 I gave a brief description of the Gallatin 

 Valley ; but inasmuch as one division of the survey took Fort Ellis as 

 its initial point again in 1872, I shall render the present account more 

 clear by presenting a resume of the geology of the valle.y. 



In beauty and fertility the valley of the Gallatin surpasses all others 



