T. W.Stanton — Fox Hills Sandstone. 177 



The minimum thickness of sandstone in the Fox Hills of 

 this ret^ion was seen in a section at liattlesnake Butte near 

 the northwest corner of T. 11 N., R. 19 E., about 15 miles 

 south of Moreau River. 



Section at Rattlesnake Butte, South Dakota. 



Feet 



1. Hard gray sandstone with Glyptostrobus Ungeri Heer, 



Taxodiam occidejitale Nevvb., Viburnwn marginatum 

 Lesq., Cornus Neioberrgi Hollick, and other plants re- 

 ferred to the " lower Fort Union "_ 25 



2. Clay shale 10 



3. Carbonaceous shale -. - 4 



4. Clay shale with some carbonaceons bands 8 



5. Carbonaceous shale Y 



6. Soft, friable yellowish sandstone with large concretions 



or indurated masses in the upper part, forming top of 

 Fox Hills _. 1 12 



7. Banded shale with a few marine Fox Hills shells 90 



This is the base of the local exposure, but in the neighbor- 

 hood dark shale referred to Pierre is seen not many feet lower. 

 The upper 5 members evidently belong to the Lance formation. 

 No. 6 did not yield any fossils at this point, but on Mud Butte, 

 about 5 miles south, the same horizon yielded the mixture of 

 brackish-water and marine Fox Hills species listed as ISTo. 5870 

 on p. 179. 



It is evident from these sections that the Fox Hills sandstone 

 is variable thl-oughout its thickness. The lower part is no 

 more constant than the upper. Its base is therefore indefinite, 

 and it is by no means certain that the top of the dark shale 

 necessarily taken as the top of the Pierre in the different sec- 

 tions represents exactly the same geologic horizon. The fauna 

 of at least the upper 100 feet of Pierre shale is essentially a 

 Fox Hills fauna, indicating shallow marine waters and differs 

 very little from that of the overlying sandstone, while it lacks 

 all the species which in other areas are especially characteristic 

 of the Pierre. 



The most common and characteristic species of this fauna in 

 the Fox Hills sandstone and the upper part of the Pierre shale 

 are as follows : 



Avicula linguiformis E. & S. 

 Avicula nebraseana E. & S. 

 G'ucidlcea s/iumardi M. & H. 

 Llmopsis striatopunctata E. & S. 

 Leda {Yoldia) evansi M. & H. 

 Nucida cancellata M. & H. 

 NuGxda planimarginata M. & H. 



Am. Joub. Sci. — Fourth Series, Vol. XXX, No. 177. — September, 1910. 



