352 HATCHER — MARINE AND NON-MARINE FvORMATIONS. [April 7, 



first differentiated by Marsh who named them the Atlantosaurns 

 beds} They have since received various other names by other 

 authors, such as the Como beds, the Morrison beds, the Beulah 

 shales, etc. Since these later names are no more appropriate than 

 the one first given there is no good reason for accepting any of 

 them and rejecting the name first bestowed on these beds by their 

 discoverer. Moreover, if stability in geological nomenclature be 

 desirable there would seem to be no better way of securing it than 

 by recognizing priority. It may be well to adopt certain rules 

 governing the formation of new names for new formations, but these 

 rules should not be made retroactive in their application, since 

 such application will not only fail to do full justice to many earlier 

 geologists but, what is of far more importance, will result in increas- 

 ing still further that synonymy already existing in our geological 

 nomenclature. 



In their typical exposures the Atlantosaurns beds consist of rather 

 fine shales. At certain localities there are in them occasional small 

 lenses of limestone with fresh water invertebrates and more import- 

 ant lenses of sandstones frequently appearing as distinct and quite 

 continuous strata. Toward the top the sandstones become more 

 frequent and the whole is everywhere overlaid by a heavy bed of 

 sandstone known as the Dakota sandstones, save where these have 

 been removed by erosion. In some places the passage from the 

 sandstones and shales of the Atlantosaurns beds to the Dakota is 

 quite abrupt, but as a rule it is very gradual, the two grading into 

 one another in such manner that it is difficult to say where the one 

 ends and the other begins Both these formations are of fresh 

 water and aeolian origin, though there is evidence of brackish 

 water or littoral deposits in a i^^f^ places near the top of the Dakota. 



The Dakota sandstones have usually by general consent been 

 considered as representing the base of the Upper Cretaceous, both 

 from their stratigraphic position beneath the Benton and from their 

 fossil flora. The latter, however, can scarcely be considered as 

 being at present at least of any special value in determining the 

 exact age of these sandstones, which in many places in the western 

 portion of the region under consideration pass insensibly below into 

 the sandstones and shales of the Atlantosaurus beds. In the region 

 near Buffalo Gap. S. D., Darton has distinguished these transitional 



1 The Hallopiis beds of the same author are disregarded here as of minor 

 importance and being usually not distinguishable. 



