30 proceedings: geological society 



in which evidence was presented that seemed to prove that the Dakota 

 sandstone originally extended uninterruptedly over the area now occu- 

 pied by the Rocky Mountains in Colorado and New Mexico. The 

 present paper presents evidence that the marine Upper Cretaceous 

 formations also originally covered this area, and that the Cretaceous 

 sediments of the Rocky Mountain region came mainly from the con- 

 tinental land mass that lay west of the interior sea during Cretaceous 

 time. That this was the course of the sediments is indicated by the 

 prevalence of sandstone, some of which is conglomerate near the 

 shores of this western continent and by the prevalence of marine shale 

 farther east; and also by the thinning eastward toward the present 

 Rocky Mountains, of sandstones such as the Mesaverde in the San Juan 

 Basin and the Ferron sandstone of Utah and western Colorado, and by 

 the corresponding thickening of the marine shales in the same direction. 



Comparison of published sections viewed in the light of personal 

 observation in the field indicates that the Cretaceous formations on 

 opposite sides of the mountains and in the intermontane basins are 

 comparable in thickness, character and stratigraphic succession. The 

 thickness of the Pierre shale, however, as reported from various locali- 

 ties, especially east of the mountains, varies by some thousands of feet. 

 This has been interpreted as due to local down-warps of the Cretaceous 

 floor with assumed corresponding up-warps in the present mountain- 

 ous area. But it is possible that these great differences in thickness 

 may be due in large part to crushing and other rock movements following 

 deposition rather than to original differences. 



It seems probable that the interior Cretaceous Basin which includes 

 the present Rocky Mountain areas was a great geosyncline in which, 

 until near the close of the Cretaceous the main movement was down- 

 ward, with minor warpings. It also seems probable that there was no 

 effective barrier in the relatively small area now occupied by the moun- 

 tains to prevent the uniform spread of sediments derived from the 

 continental mass west of the Cretaceous sea. If this hypothesis endures 

 the test of future investigation it should lead to a readjustment of cer- 

 tain correlations that now seem discordant and should indicate definitely 

 that the Cretaceous-Tertiary unconformity represents a clearly defined 

 period of erosion, for the Cretaceous sedimentary rocks must have been 

 removed before the pre-Cretaceous rocks could supply the pebbles 

 found in the basal conglomerates of the Tertiary. 



E. W. Shaw: A study of the Lafayette at and near the type locality. 

 Although the surface of Lafayette County, Miss., and much adjoining 

 territory is immediately underlain by a thin wash-creep-residuum 

 mantle made up principally of material derived from underlying strata 

 but containing certain elements not found in them, it is concluded from 

 recent studies that no formation such as the Lafayette is conceived and 

 described to be is present in the county from which it was named. On 

 the other hand, such a formation is present in the county just west of 

 Lafayette and in other places in the Coastal Plain, and it therefore 

 appears probable that the concept Lafayette, with certain modifica- 

 tions, will be retained. 



