318 G. H. Eldridge — Method of grouping the 



the two horizons, as seems to be the case in Texas and New 

 Mexico. In passing from the Niobrara group, however, into 

 the succeeding rocks above, in which great numbers of fossils 

 occur, not a single species, as far as known to the writer, has 

 yet been found identical with any form known from either 

 of the three divisions below. In addition to this, the upper 

 surface of the Niobrara beds is, at several places on the 

 Missouri, seen to have been eroded into irregularities, or 

 depressions, previous to the deposition of the succeediug Fort 

 Pierre group, thus giving additional evidence that some kind 

 of a physical change (perhaps slight) occurred between the 

 deposition of the latest portion of the Niobrara division and 

 the first of the Fort Pierre beds." 



Finally, the views of Dr. George M. Dawson of the Can- 

 adian Geological Survey are also interesting in this connection. 

 In a personal letter to the writer he remarks : " I agree fully 

 with your main proposition as to the grouping together of the 

 Fort Pierre and Fox Hills. It has proved impossible in our 

 western region — from the 49th parallel to the Peace River — to 

 make a satisfactory paleontological division, and in the northern 

 part of Alberta sandy zones containing an essentially Fox 

 Hills assemblage of fossils have been found even toward the 

 base of the Pierre shales as locally developed.'' 



With regard, however, to the lower portion of the series of 

 formations under discussion, Dr. Dawson remarks : " Nothing 

 which can really be said to represent the Niobrara has been 

 .found north of the 49th parallel to the west of Manitoba 

 region. While the Belly River series may in part represent 

 the Niobrara, this cannot yet be proved ;'' — and referring to 

 the rank of the Dakota and Laramie : " The Belly River series 

 for identical reasons, must be allowed to nold a similar inde- 

 pendent position, representing, as it does, entirely differeut 

 conditions of deposition from the formations above and below 

 it." Again, however : " As an exception to the above, I may 

 mention that in the Manitoba region, from the existence of 

 blending between the Niobrara and Fort Pierre formations, it 

 has so far been found impossible to there establish a dividing 

 line between them." 



Dr. Dawson, in concluding, remarks that, so far as his 

 knowledge goes at the present time, he does " not consider the 

 restricted Colorado group a useful division of the Cretaceous 

 over the western portion of the country of the plains north of 

 the 49th parallel." 



Reflection upon the above facts presented by Dr. Dawson 

 leads to the following observations : first, — in regard to that 

 portion of them which relates to the Belly River series or its 

 possible equivalent the Niobrara, — although the present state 



