GEOLOGICAL SUEVEY OF THE TEEEITOEIES. 23 



down from the mountains on each side, and seem to give shape to the 

 valleys of the multitude of little streams. We have here and there an 

 exposure of the rocks, which are undoubtedly eruptive, and present the 

 appearance of stratilied deposits. They are arranged in more or less thin 

 layers, some of which sound under the hammer like clink-stone, and are 

 quite compact. Sometimes the breakage-joints, or cleavage, are vertical 

 in a single layer, but from their external appearance I would suppose 

 the blufi's of vertical rocks were a dark-gray marly limestone, charged 

 with fossils. There is also a good deal of uniformity in its composition, 

 the only dilference being that some of it is more compact than others. 

 The eruptive material in this valley assumes a variety of form ; some of 

 it has a black, opaque crystalline appearance, like obsidian ; then a sort 

 of sandstone, easily decomposing, or, as it were, exfoliating ; then a sort 

 of lava, or slag ; then a vesicular trachyte. There are also veins of 

 quartz, sometimes ribbon-like, one-fourth of an inch wide. The greater part 

 of these rocks, however, would seem to have been melted or heated under 

 comparatively little pressure. These rocks predominate, and, indeed, 

 comprise almost the only rocks on the western slope, and therefore it 

 may be called a basalt country. Many of these rocks seem to yield very 

 readily to the decomposing agencies of the atmosphere, and furnish en- 

 tirely the soil of the valley, which is quite black and fertile, sustaining- 

 a luxuriant growth of vegetation. The streams that issue from the 

 mountains are very numerous, the water pure as crystal, and the val- 

 leys clothed with rank herbage; but the timber, which fringes the little 

 streams here and there, is very scarce. There are also many beautiful 

 springs and lakes. 



" June 20. — We i^assed up the valley of the Lake Fork and crossed the 

 dividing crest of the mountains to the Madison Fork of the Missouri. 

 High hills of eruptive rock surround us on every side, with now and then 

 small patches of limestone along their sides, inclining at various angles. 

 There are, also, mica schists, talcose slates, and quartzose limestones often 

 underlying the layers of eruptive material, and conforming to them in 

 inclination, which is from SC^ to 60°. Many of the ridges are 2,000 feet 

 or more above us, and are covered with snow. The Low or Eaynold's 

 Pass is like a lawn — smooth and covered with grass, with a large super- 

 ficial deposit composed of the rocks in the vicinity. It is plain that the 

 eroding agency of water has had its effect in smoothing this pass, though 

 it has not formed it. It is undoubtedly due, to a great extent, to a break 

 in the continuity of the elevatory force. The mountains here do not 

 seem to follow any fixed lines of fracture, or in fixed directions, but to be 

 a series of protrusions, forming, in many instances, a continuous line 

 for a great distance; but the irregularity of the outline of the crest is 

 due, to a great extent, to the irregularity of the force along the line of 

 continuity, though a small portion may be dae to atmospheric agencies. 

 The facts above stated are true from the fact that the different strata of 

 sedimentary rocks, which must, prior to the upheaval of these ridges, 

 have covered the surface, lie in regular order of sequence outward from 

 the ridges. We have every variety of volcanic rocks and metamorphic 

 conditions. Washed out of the Madison caiion and scattered over the 

 terraces along that stream are every variety of granitoid rocks, mica 

 slates, hornblende, &c. There is every variety of these rocks, depending 

 upon the greater or less predominance of some constituent, and dissem- 

 inated through the rock are seams of white quartz. None of the red 

 feld spathic rocks which so prevail in the Black Hills are seen in this 

 region. Along the rivers is a series of terraces whicli are covered with 

 bowlders, slightly worn, exhibiting the rock-character of the mountains 



