﻿24 INTRODUCTORY. 



of the lower cyead horizon on the eastern side of the Black Hills in the Piedmont 

 region. These cores are composed of indurated materials best accounted for as 

 having accumulated along the course of deep-seated and probably hot and silica- 

 laden waters as these made their way to the surface. But that any such extensive 

 thermal action occurred as may be observed in the Yellowstone Park at the present 

 time, where in the vicinity of the geysers and hot springs wood may be frequently 

 seen in various initial stages of silicification, would seem unlikely; for such 

 chemical activities woidd probably leave indubitable evidence of their former 

 presence at many points. However, it is noteworthy that on the southern side of 

 the Black Hills, hard by the great Minnekahta c} - cad locality, are some layers of 

 chalcedony or onyx, a foot or more in thickness, interpolated in the shale at the base 

 of the Atlantosaurus beds ; and these, considered together with the siliceous cores 

 just mentioned, certainly imply some abundant and unusual local sources of silica 

 in the Black Hills mountain knot region, as girdled by the lower and the upper 

 and more important cyead horizon which has yielded the magnificent Minnekahta 

 and Black Hawk series of trunks. 



Aside from the sources of silica mentioned, thermal action on a siliceous matrix 

 is a remaining and perchance the most likely possibility. While in both the Black 

 Hills cyead horizons there is much similarity in the preservation of the cycads and 

 the accompanying stems of trees, the nature of the trunk matrix and actual con- 

 ditions of deposition are best known in the upper horizon. It has been the writer's 

 good fortune to discover numerous cycadean trunks, accompanied by much silicified 

 wood, in both the main localities of the upper horizon. At Minnekahta the cycads 

 are distributed through sandstone 10 or more feet in thickness which caps a highly 

 characteristic bed of blue clay 30 or more feet thick, and thus forms a well-marked 

 hill-top, in this instance so fortunately spared by the vicissitudes of time. The 

 upper part of the 10 feet of sandstone here referred to is, in places, of quite light 

 to flesh color, and the silicified stems from it are likewise of lighter color and often 

 more or less chalcedonized. In the lower part of the bed the sandstone is highly 

 colored, its yellowish hue, due to iron oxides, often being visible for long distances, 

 especially so 4 miles to the east along the prominent outcrop on the precipitous 

 walls of Hell Canyon. There are irregularly or locally interpolated in this lower 

 portion many thin layers of clay, and from this mixture of sand and clay most of 

 the trunks of darker color seem to have come. In some cases the trunks are 

 embedded in the sandstone layer as interspersed with clay seams, but actually rest 

 on the heavy day layer beneath. Evidently this clay formed the bottom of a fresh- 

 water lake, along the shores of which patches of cycads and great groves of 

 Araucarias were prominent in the forest facies ; and with some change in estuarine 

 conditions the clay bottom was in some places thickly strewn with cyead trunks 

 and a few Araucarian logs, in others with numerous logs and occasional cycads 

 which were quickly covered over by an inrush of sand, carrying with it further 

 scattering cycads and a few tree trunks, all of which were doubtless early silicified. 

 Aside from the possible action of silica-laden waters already suggested, this covering 

 of sand may have furnished the source of silica, as the clays have not been found 



