180 GEOLOGY OF THE DENVER BASIN. 



dark in color, and very hard, while lying in a soft, friable sand rock, 

 easily disintegrating and of yellowish-brown color. In form the nodules 

 are more frequently lenticular than round, but almost perfect spheres have 

 been seen in some cases. As a rule certain layers carry many nodules, 

 and occasionally they adjoin each other so closely as to make up the 

 greater part of a given stratum, or they may he united, and as an extreme 

 of this development a given layer may he found to possess the color and 

 hardness of a nodule for several yards with quite uniform thickness, though 

 alwavs ending with round outlines showing the real formal relation. 



A complex of layers in which such lenticular masses are abundant 

 rarely fails to make itself known by good outcrops. On Murphy Creek 

 are several particularly hue exposures of such strata, and on the High Line 

 ditch, between Cherry Creek and the Platte, such outcrops are numerous. 

 In the latter case, the bed of the ditch for a short distance below an exposure 

 of nodular masses is covered by the nodules washed out. Similarly, on the 

 dry creeks of the region south and southeast of Denver, the appearance 

 of (lark nodular fragments in the stream bed surely indicates good expo- 

 sures not far upstream, cut out by the floods of the rainy season. Good 

 examples of the nodular masses may also lie seen on the west bank of the 

 Platte opposite Overland Park. 



One feature of many outcrops of the Denver strata seen upon the 

 plains is likely to be at least temporarily misunderstood by anyone not 

 thoroughly acquainted with the developments in Table Mountain. This 

 feature is the irregular, unconformable contact so frequently seen to exist 

 between a conglomerate or grit layer above ami a clay or shale below. 

 Often the distinctive eruptive character of some of the pebbles and frag- 

 ments of the coarser bed may he seen at a glance, while in the clay below 

 nothing showing its true position is visible, and one is inclined to wonder 

 if the actual base of the Denver may not be exposed, the clay belonging to 

 the Arapahoe or Laramie. < Jften the unconformability is very marked, and 

 unless adjacent exposures show similar relations at other levels, or reveal 

 characteristic Denver sandstones below the clay, the true relationship may 

 not lie clear. The changes in conditions of sedimentation which give rise to 

 such stratigraphical relations of consecutive beds were, however, common 



