158 GEOLOGY OF TUE BLACK HILLS. 



The next detailed examination of the formation was on Amphibious 

 Creek, near the canon by which it passes the Cretaceous ridge or " foot- 

 hills," The following section was measured: 



Cretaceous {395 feet exposed). 



Feet. 



16. Sandstone, capjnng hill; massive, white, stained yellowish and reddish. 300 



15. Unexposed 25 



14. Sandstone, soft and massive ; pinkish in color 70 



Jura [234: feet). 



13. Mostly concealed, a talus overlying soft rocks which at base appear to be a 



white or reddish, soft, argillaceous sandstone 105 



12. Sandstone, light red, soft and incoherent, almost a sand 55 



11. Sandstone, greenish and calcareous, thinly bedded 20 



10. Sandstone, white or greenish, in thin layers (8-10 inches), with many ripi)le 



marks 20 



9. Shale, red and gray, and shaly sandstone 8 



8. Shale, red and gray 5 



7. Sandstone, red and pink ; soft at bottom, hard above ; with ripple marks ... 8 



6. Sandstone, yellowish, greenish, reddish, a thin brecciated band 10 



5. Sandstone, white 3 



Red Beds {255 feet exposed). 



4. Clay, deep red 60 



3. Gypsum ^. 10 



2. Clay, red, with gypsum . . 175 



1. Limestone, pink or purple , 20 



The Jura was recognized in this section by certain of its beds, but iu 

 the absence of fossils it is hard to say where the separating lines should be 

 drawn. Taking the demarkation here made the formation has a total of 

 234 feet, a much greater thickness than was found on Red Cafion Creek, 

 twenty-five miles to the westward. It will also be noticed that the arena- 

 ceous materials are here more than usually predominant. 



At Buffalo Gap, the pass by which Burntwood Creek breaks through 

 the foothills, the exposures of the Jura are very imperfect. On lower 

 French Creek and on lower Spring Creek observations were equally unsatis- 

 factory. Occasional outcrops of the yellow and drab clays and soft sand- 

 stones were seen, but no continuous section was an3^where attainable, and 

 the rocks appeared to be entirely barren of organic remains. Near Rapid 

 and Box Elder Creeks and north of the latter, frequent exposures of the 



