201 



is an expansion of tbe valley' of the Missouri, in which these deposits 

 occur again to a considerable extent. Indeed, tbe river-valleys in the 

 vicinity of the mountains may be regarded as having been made up of 

 chains of small lakes, interrupted here and there by acafion or a mount- 

 ain-range. These lake-basins, large and small, are, for the most part, 

 the resnlt of erosion. In the general elevation of the countr^^, when the 

 crust was broken up, there were numerous depressions, in which tbe 

 waters flowing down from the more elevated area may have accumu- 

 lated ; but it is plain that they have been slowly scooped out by water. 

 The general fineness of the sediments would seem to indicate compara- 

 tively quiet waters during this Lacustrine period ; butthe original basins 

 may have been worn out by the combined action of water and ice. 



The lake-basin near the junccion of the three forks extends up the 

 valley of each of the streams for a considerable distance. It covers the 

 entire valley of the Gallatin and Madison and the interval between the 

 rivers for more than twenty miles above their junction. In the Jefferson 

 Valley, Lacustrine deposits are cut oft" about ten miles up the river by 

 a high ridge of limestone. This lake deposit may be estimated to cover 

 an area of about one thousand square miles. In Plate I, showing the 

 junction of Ihe three forks, we see at the right the nearly horizontal 

 strata of Carboniferous limestone, which forms one of the walls of the 

 canon. This ridge of limestone seems to have obstructed the flow of 

 tbe waters until the channel was worn through. It must have been 

 much higher than at present, but was gradually worn away, the waters 

 covering a broad surface until they centered in this narrow channel. 



In the lower section on the Gallatin Eiver, the Carboniferous lime- 

 stones are exposed from beneath the lake-deposits in a nearly vertical 

 position. The deposits are much scattered over the surface as we pass 

 up the stream, but the limestones crop out everywhere, showing the 

 character of the basis-rocks. These limestones all present unmistak- 

 able marks of having been worn down to their present condition. The 

 modern lake-deposits all hold a horizontal position, or nearly so, and 

 fi^om this fact we infer that there has been no very marked change in 

 the general elevation of the country since their deposition. In Plates 

 VIII and IX, there is a connected series of four sections along the im- 

 mediate margin of the Madison. Eiver, extending from its month to the 

 southern border of the basin near the foot of the mountains, a distance 

 of thirty miles. Section jSTo. 8 shows the east wall of the channel of 

 the Madison, as it has been worn oat of the Lacustrine deposits. Com- 

 mencing at the left of Xo. 4, the horizontal strata may be followed on 

 sections 3, 2, and 1, until the granites of the higher mountain-ranges 

 are uncovered by erosion. 



The system of terraces is well exhibited, as well as the n)yriads of 

 small gullies, or gorges, which are worn out of the sides. In the lower 

 terrace of section No. 1, the vertical schists are seen cropping out from 

 beneath the Lacustrine beds. These Lacustrine sediments are composed, 

 for the most part, of silica and alumina, the former ])reLlominating. 

 There is some lime in portions, doubtless deiived from the wear of the 

 Carboniferous group. The prevailing color is light brown-gray or nearly 

 white, perhaps in some instances a cream color. 



There is a remarkable similarity in the color as well as th(; composi- 

 tion of these dei)osits all over the West, in Montana, Oregon, New 

 Mexico, or Texas. Tiie forms which result from weathering are m ich 

 the same, even though of a dift'erent age. The Santa Fe marls, wliich 

 cover so broad an area in New Mexico, the Arkansas marls, in tbe val- 

 ley of the Upper Arkansas River, the Loup P'ork group, on the Loup Fork 



