216 GEOLOGICAL SUEVEY OF THE TERRITORIES. 



From this point, there is a slope with occasional outcrops of red 

 shales and greenish sandstones, all rather coarse. These no 

 doubt continue to the red-beds (Triassic ?), above which is Cretaceous 

 No. 1, to which I referred above as crossing Tarryall Creek above Ham- 

 ilton. Beyond this is a hill of the same material as that resting on the 

 same bed (Cretaceous No. 1) on the north side of the creek. This slopes 

 to the park, where the Tertiary sandstones appear. The distances in the 

 section given above are all estimated. For want of time, I was unable to 

 carry the section any farther to the eastward, but the sections made in 

 the park at points farther southward comprehend it. Silver Heels is 

 drained on the south by the branches of the South Platte, and is hol- 

 lowed out, leaving the high ridge on which the section is made standing 

 out prominently. The volcanic layers seem to be intruded masses which 

 have so changed the sedimentary beds and hardened them that they 

 have been able to resist the eroding influences, and the result is this 

 prominent ridge. Leaving Hamilton, and still keeping close to the edge 

 of the mountains, in going around the northwest rim of the park, we 

 notice after crossing Tarryall Creek that the Tertiary formations extend 

 to the edge of the mountains, which -here are volcanic. Near McLaugh- 

 lin's ranch is a coal-mine. This was visited by Dr. Hayden, and will no 

 doubt be fully treated of by him. The Tertiary beds here are so cov- 

 ered with drift that they are almost entirely concealed. Following the 

 road southward we pass around the edge of a trachytic hill, and come 

 across cretaceous shales belonging in all probability to the Fort Pierre 

 group, (No. 4.^ They are found in a valley between the hill just referred 

 to and a low volcanic ridge that runs out into the park. Crossing this 

 ridge we again pass over the shales. Here is a well-marked terrace on 

 a small branch of Tarryall Creek. It is about 100 feet in height. Ter- 

 races are to be seen on almost all of the branches of Tarryall in the 

 northwestern part of the park. Just south of the terrace referred to is 

 a long volcanic ridge, the course of which is almost due north and south. 

 I have already spoken of this ridge, which is shown in the profile across 

 the northern end of the park. The following section was made from the 

 Platte Eiver eastward to Trout Creek, about five miles north of Fair 

 Play, and is shown in Fig. 1, Plate VI. The length of the section No. 9, 

 given below, is about six miles. The distances and thicknesses are es- 

 timated, the average dip being about 30° toward the eastward. 



Section No. 9, 



a. 1. Starting from the Platte Eiver, the point a in the illustration, 

 we have the valley covered with drift to the edge of the hills, a dis- 

 tance of about a mile. We then begin to ascend the hills, which are 

 well timbered, concealing the beds. From the character of the debris, I 

 think there is volcanic rock beneath. The first exposure we met with 

 furnished — 



b. 2. Eed sandstone shales : thickness, 400 to 500 feet. 



Then followed : 



3. Gray shaly sandstone. 



4. Brownish limestone. 



5. Eed shales. 



6. Very coarse white micaceous sandstone. These beds are indi- 



cated by very indistinct outcrops, and are from 500 to 600 feet. 

 Then we have next — 



c. 7. Limestone mostly of a grayish color or grayish-blue, in some 



places brownish, compact in some layers, and then again ir- 

 regular, with seams of calcite. Thickness, 6 feet. 



