204 



worn out, the aqueous forces operating witli great power, and transport- 

 ing the sediments down into the Miocene lakes of the i^Iaius; second,, 

 when the waters set bacli 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 op- 

 erating were those of water and ice, or what is understood as the Gla- 

 cial 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 sliown in Plates VIII and IX. The 

 deposits of this third i)eriod are immense in many localities, concealing 

 all the basis-rocks and giving the peculiar slopes to the base of the 

 mountains as they graduate into the plains. In the mountain-valleys, 

 as, for instance, in the oval lake-basins between the lower and second 

 cailons of the Yellowstone, high bowlders of granite, 25 feet in diameter, 

 have been brought down Irom the mountains and lodged on the surface 

 of the terraces at different elevations above the present river-bed. The 

 remnants of the layers or Hour of basalt that, since the deposition of 

 the Lacustrine sediments, have flowed over the valley, exhibit abundant 

 proofs of glacial action iu the very smooth crust, which even now is like 

 enamel. On this basaltic floor, 50 to 100 feet above the bed of the Yel- 

 lowstone 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 bed-rock, 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 surfoce at 

 the present time. The first period must have commenced either during 

 or at the close of the ]\liocene 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 all these periods. During the Mio- 

 cene and Pliocene periods, the animal remains which were preserved in 

 theLacustriue 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 de- 

 crease of tonperature, 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 Mon- 

 tana. It is intended simi)ly to render more plain, if possible, the mean- 

 ing of the beautiful pictorial sections which accompany it. As repre- 

 sentations of the varied forms of the scenery in Montana, and, indeed, 

 to a limited extent, in any other portion of the West, they are unsur- 

 passed. 



Some of the plates have alread3" been described incidentally in these 

 notes. Plates II and III are closely connected, though not intended to 

 be absolutelj' parts of the same section. They were sketched from dif- 



