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Show 204 worn out, the aqueous forces operating with great power, and transporting the sediments down into the Miocene lakes of the plains; second, when the waters set back up toward the sources of all the mountain-streams, producing that quiet condition in the lake- basins that enabled the finer sediments to settle upon the bottoms of the Pleiocene lakes, with the strata regular and horizontal as we find them at the present time; the third period may be regarded as the one when the local drift- deposits were made. This period was one iu which the forces operating were those of water and ice, or what is understood as the Glacial era of the West. The channels of all the rivers were deepened, and the Lacustrine deposits were in part removed and carried down into the plains. It was during this period that the terraces and fine sections were here formed, which are so admirably shown in Plates VIII and IX. The deposits of this third period are immense in many localities, concealing all the basis- rocks and giving the peculiar slopes to the base of the mountains as tbey graduate into the plains. Iu the mountain- valleys, as, for instance, in the oval lake- basins between the lower and second canons of the Yellowstone, high bowlders of granite, 25 feet in diameter* have been brought'down troin the mouutaius and lodged on the surface of the terraces at differeut elevations above the present river- bed. The remnants of the layers or floor of basalt that, since the deposition of the Lacustrine sediments, have flowed over the valley, exhibit abundant proofs of glacial action in the very smooth crust, which even now is like enamel. On this basaltic floor, 50 to 100 feet above the bed of the Yellowstone Eiver, many of these large bowlders now rest. Worn masses of smaller size are scattered thickly all over the surface. The greater portion of the local drift is composed of rounded bowlders, varying from a few inches to several feet in diameter, with a small proportion of sand and clay mingled with it. It is at the bottom of this deposit, near what is termed by the miners the bedrock, that placer gold is found. It is not pretended that the three periods named above can be defined by arbitrary lines by any proofs that have been left on the surface at the present time. The first period must have commeuced either during or at the close of the Miocene era; and there is nothing that indicates any remarkable abrupt break in the sequence of events up to the present time. There was undoubtedly a constant variation in the intensity of the forces that were in operation during ail these periods. During the Miocene and Pliocene periods, the animal remains which were preserved in the Lacustrine sediments indicate a comparatively mild climate. The cold period must have approachd gradually, reaching its greatest intensity, covering the mountains and filling the valleys with immense bodies of snow and ice, which slowly melted away, leaving the peculiar lake- basins at the sources of the mountain- streams and morainal deposits,. as proofs of its former existence. It is probable, also, that it was during the decrease of temperature, while the waters were subsiding to their present condition, that the terraces were formed. It is not the purpose of this-article to present anything like a complete view of the geology of Montana. It is intended simply to render more plain, if possible, the meaning of the beautiful pictorial sections which aocompany it. As representations of the varied forms of the scenery in Montana, and, indeed, to a limited extent, in any other portion of the West, they are unsurpassed. Some of the plates have already been described incidentally in these notes. Plates II and IU are closely connected, though not intended to be absolutely parts of the same section. They were sketched from dif- |
