Page:Popular Science Monthly Volume 79.djvu/426

422

sediments in the northeastern part of the field are narrow dikes and sills of diabase and basalt which are either Tertiary or post-Tertiary in age. The morainal deposits extend beyond the present limits of glaciation only a few miles.

The Tertiary sediments have been divided by Dr. Martin into three formations, namely, the Stillwater, the Kushtaka and the Tokun. The Stillwater is the oldest formation, and consists chiefly of sandstone and shale with a thickness of about 1,000 feet. The Kushtaka overlies the Stillwater conformably and is that part of the Tertiary which contains the beds of coal. It has a thickness of about 2,000 feet made up of coarse arkose, sandstone, shale and beds of coal. Complete sections