﻿PRESERVATION AND EXTERNAL CHARACTERS. 25 



to consist in volcanic materials or to contain diatoms or siliceous spicules ; but 

 what in such case were the exact conditions permitting chemical activity is difficult 

 to conjecture. Those trunks lying on the clay surface are usually very dark in 

 color. Those from a little higher up in the sandstone are somewhat lighter or 

 yellowish on the outside, and those from the highest points in the bed the lightest 

 in general coloration. 



In the Black Hawk locality, 60 miles northeast of Minnekahta, as has been 

 stated, the conditions are very much the same as at Minnekahta, except that there 

 is less clay in and near the cycad-bearing horizon. The sandstones in which the 

 cycads and numerous silicified logs of immense size occur are not always so much 

 iron-stained, and, as might be expected, the embedded silicified plants show less 

 tissue differentiation. Many of the cycad trunks in particular seem to have been 

 subjected to the steady action of an abundant source of nearly iron-free silicic-acid 

 solutions, resulting in some cases in a close approach to chalcedonization. In fact, 

 there may be seen in the specimens of this locality nearly every gradation from 

 complete differentiation of cell structure, or histometabasis, to structureless chalce- 

 dony casts. 



The conditions of silicification in the Freezeout Hills of central Wyoming 

 varied but little from those of the lower cycad-bearing horizon of the Black Hills. 

 This much is certain from the great similarity in general surroundings, as evidenced 

 by both the character of the strata in which the cycads are embedded and the other 

 fossils present. Although these two localities are quite 200 miles apart, there is 

 here a case in which there is general coincidence in all features of form and preserva- 

 tion, instead of the variation seen in most other as widely separated localities. 



Finally, it is to be noted that, although the conditions of preservation may 

 have varied considerably in the three main North American cycad regions, as just 

 described, in none of them do they seem to have made possible the silicification 

 of mature fronds in connection with or even adjacent to the trunks which bore 

 them. This lack of foliage preservation would still leave a nearly unbridgeable 

 gap in our knowledge of these plants were it not for the abundant silicified non- 

 emergent young fronds often preserved in such marvelous detail in the Black 

 Hills specimens, as described at length in Chapter V. In general, where trunks, 

 fronds, and fruits are represented in the same beds, as in the case of the William- 

 sonias of Hawkser and Ruuswick, it is only as casts or impressions, though we 

 do have, as previously mentioned, the single instance of the Pterophyllum leaves 

 attached to their silicified trunks in some of the Indian beds. 



But, as bearing on future field work in the Black Hills, it is to be borne iu mind 

 that, despite the number of trunks already obtained as eroded out in the slow course 

 of time, little is known of their matrix, the few trunks observed in situ by the 

 writer being the only ones so found thus far. In the case of one of these, however, 

 traces of pinnules were present in the surrounding clayey saudrock ; so that it is 

 probable that adequate excavation would occasionally reveal impressions of fronds, 

 although such can never be expected to resist the action of frost while weathering 

 out of an imperfectly consolidated matrix. 



