316 G. H. Eldridge — Method of grouping the 



prominent forms are mainly included in the genera Inoceramus. 

 Cucullcea, Nautilus, Placenticeras, B acuities and Scaphites. 



At the summit of the Fort Pierre, between it and the Fox 

 Hills, occurs the second marked zone of transition in the great 

 Cretaceous series of formations. In passing upward, it is first 

 recognized only in a very slight change in the character of the 

 sediments laid down, their composition passing from that of a 

 pure clay to one in which there is a minute but distinguishable 

 amount of fine arenaceous material. In the overlying strata 

 the arenaceous constituents are found to further increase until, 

 finally, the opposite condition is reached, in which we have a 

 large predominance of sandy material with but a compara- 

 tively small proportion of purely argillaceous matter, — a con- 

 dition which shortly afterwards becomes normal for the main 

 mass of the Fox Hills shales. Limestones, similar to those 

 occurring below, are present throughout this zone and extend 

 even well up into the formation above. Fossils also occur, but 

 the special mark in the life of the zone, is the sudden increase 

 in the members of the genus Mactra, a genus which has only 

 occasionally been met with below, but which, from this upward, 

 is present in innumerable quantities. 



The Fox Hills. — This formation, the closing member of the 

 marine Cretaceous, embraces a thickness of strata varying 

 between two or three hundred and one thousand feet, according 

 to locality. It is essentially a formation of arenaceous shales, 

 of fine material, soft and friable, but still having in an impor- 

 tant degree an admixture of argillaceous matter, which occurs 

 both disseminated amidst the fine sand and in occasional bands 

 interstratified with those more purely arenaceous. Limestone 

 concretions, similar to those described for the Fort Pierre, 

 occur here and there through the shales, though perhaps in 

 somewhat fewer numbers. At the summit of the series lies a 

 prominent and most important bed of sandstone, of universal 

 occurrence, having a uniform thickness of about fifty feet. 

 The entire formation has a decidedly yellowish cast, the 

 capping sandstone especially so, though frequently modified by 

 a slight tinge of green. 



The sandstone referred to derives its importance from the 

 position it holds as capping the great series of marine Cretace- 

 ous sediments ; from the decided differentiation from its ma- 

 terial, of that of the basal sandstone of the Laramie which 

 immediately succeeds ; and from the value of its upper stratum 

 as a line of demarkation between the two formations. The 

 last feature may be ascribed to the abundance and the distinct 

 character of the fossil forms that occur at its very line of union 

 with the overlying Laramie, none of which forms are ever found 

 above, and but few of which are met with, in numbers, below. 



