190 Bulletin American Musettm of Natural History. [Vol. XIV, 



clines that the American Museum parties have worked, where the 

 dip is 14-18°. The north sides of these folds stand too nearly 

 vertical to be worked. Plate XXVI, Fig. 3, shows a section re- 

 constructed to show the appearance of these folds had no erosion 

 taken place. The position of this section is shown on Plate 

 XXVI, Fig. 2, by a line from H to L running N. 25° W. The 

 line starts on the Little Medicine River, about one and one half 

 miles below the mouth of Sheep Creek. For ten rods on either 

 side of the river all rocks are covered by river deposits. At the 

 locality c, a collection of fossils ' showed the presence of the 

 Cretaceous (Fort Pierre); while within three rods to the south 

 undoubted basal beds of the Jurassic ^ are found. A fault is 

 thus indicated. While in the field I did not trace it out, so can- 

 not speak in regard to its extent. The Jurassic is here inverted 

 so that its basal member is uppermost. Between the Jurassic and 

 the typical Triassic occurs a bed of creamy white sandstone, 40 

 feet thick, the same as is found in the Freezeout Hills, but this 

 sandstone is lacking on the exposures situated further south. 

 The Jurassic dips 60° S. (strike N. 53° E.), but proceeding south 

 the dip quickly becomes vertical, changes to the north, and 

 gradually diminishes till in the midst of the Triassic it becomes 

 horizontal. The strata soon dip to the south increasing to about 

 14°, at which angle most of the rocks, as far as the Fort Pierre 

 bed, dip. 



At G the section changes its course to N. 5° W., crossing 

 the Jurassic of the south side of the Medicine anticline. The 

 Jurassic beds are here 274 feet in thickness and are overlaid by 

 the coarse sandstones of the Dakota to the extent of 275 feet. 

 Above these in turn lie the Fort Benton shales, the lower 80 feet 

 of which consist of dense sandy limestone; while the upper 320 

 feet are thin bedded argillaceous shales, characterized by abun- 

 dance of teleost fish scales. Above the Fort Benton are about 800 

 feet of slate-colored clay in which the stratification is obscure. 

 On account of their soft texture these clays are easily eroded and 

 the exposure is usually occupied by a long ' draw.' On weather- 

 ing the clay turns black. As this clay overlies the Fort Benton 



* The collection included Inoceramus^ GryJ>heea, Liicina subundata H. and M , Tellina, 

 cf. scitula M. and H., A nisomyon shumardi W., A . Patellaformis W., and Scaphites war- 

 rent M. and H. 



'^ The Jurassic layers carried Belem7iitt's denstis M. and B. curtus L. 



