INVERTEBRATE EVIDENCE 339 



Invertebrate Evidence ^ 



In ninety-nine one-hundredths of the area over which the dinosaur- 

 bearing beds are distributed there are either no invertebrates at all or 

 they are fresh-water forms, which are generally recognized as of little 

 value in fixing age. Marine invertebrates have come into this discussion 

 from limited areas in North and South Dakota, where it is claimed they 

 have been found in or above the Lance. 



In North DaJ^ota from the Cannonball Eiver northeast to Heart Eiver, 

 a distance of perhaps 75 miles, is a long, narrow, somewhat irregular area 

 of sandy shales and sandstones from 200 to 300 feet in thickness which 

 contains a considerable marine fauna. This has been called the Cannon- 

 ball marine member of the Lance formation. Its proposers, Messrs. Win- 

 chester, Hares, and Lloyd, of the United States Geological Survey, de- 

 fined its top as the highest point at which marine invertebrates have been 

 found, and its base as the highest point at which dinosaurs occur. It was 

 regarded by its proposers as a lens in the Lance formation, and is con- 

 sidered in whole or in part as the marine equivalent of a non-marine, 

 coal-bearing horizon to which the name Ludlow lignitic member of the 

 Lance has been given. 



Three possible explanations have been advanced by different geologists 

 to account for the presence of this Cannonball member: (1) That it is a 

 lens in the Lance, in which case it must have resulted from a temporary 

 invasion of the sea after the inauguration of Eocene time; (2) that 

 it is an erosion remnant of Fox Hills, surrounded by and projecting 

 through the Lance, or (3) that Fox Hills time continued through Lance 

 to Cannonball time. 



The first of the alternative explanations — that the Cannonball is above 

 or a lens in the Lance — was the one adopted by the original namers of 

 the member, is still entertained by them, and has been accepted by the 

 United States Geological Survey."^ This means that the Cannonball is 

 separated from the Fox Hills by 400 feet or more of Lance beds — that is 

 to say, that marine conditions similar to those of the Fox Hills were re- 

 stored for the short interval of Cannonball time after the deposition of 

 the fresh-water Lance beds. It has in its favor the following points : (1) 

 Its position is above the Lance; (2) it is not known to be structurally 

 connected with the Fox Hills; (3) its fauna, though differing some- 

 what, is apparently most closely related to that of the Fox Hills; (4) it 



« See Knowlton, Proc. Wash. Acad. Sci., vol. 11, 1909, pp. 226-228. 

 ' Lloyd: The Cannonball River Lignite field, North Dakota. Bull. 541G, U. S. Geol. 

 Survey, 1914, pp. 1-51. 



