﻿22 INTRODUCTORY. 



and thin sections ; for anyone who examines macroscopically a collection of quite 

 1,000 fossil cycads must soon acknowledge himself well-nigh helpless in an attempt 

 to arrange them in appropriate species with any full degree of certainty, without 

 such aid. As macroscopic examination is, however, always the prelude to more 

 exacting investigation, it is primarily necessary, when studying such fossils 

 exteriorly, to constantly recall what the conditions and limits of preservation are, 

 in order to fairly estimate the relative value of the macroscopic characters present. 

 It is also of especial interest in this connection to note, in passing, the types and 

 conditions of preservation in the three main cycad localities of North America ; 

 for although, as just explained, there are many individual differences in the case of 

 trunks from the same locality, it is very interesting, after one has become familiar 

 with the cycads of many localities, to note certain characteristic features which, in 

 the majority of specimens, show at a glance the particular source whence they have 

 probably come. No one would be likely to mistake the " bird's nest " cycads of 

 the Isle of Portland, although cycads with very similar decayed or crushed summits 

 are also common elsewhere, a few occurring amongst the usually far more sym- 

 metrical and completely preserved trunks from Minnekahta, South Dakota. The 

 group from the Potomac formation of Maryland, with some " crows' nests," is like- 

 wise a very characteristic one. So are the much-crushed but otherwise beautiful ly 

 silicified trunks from Dakota and Wyoming, constituting the genus Cycadclla. 



The single cycad (Cycadcoidca nigra Ward) from near Boulder, Colorado; it 

 may be mentioned, shows, together with beautiful differentiation of its tissues and 

 denseness of silicification, a certain toughness in grinding and a jet-black color, 

 such as are seen in no other specimen known to me. Yet in form aud in various 

 minor details, particularly in the numerous fructifications in the axils of the leaf- 

 bases, this trunk agrees in the most striking manner with certain Maryland aud 

 Italian trunks. Indeed, in so far as macroscopic features go, it has always been a 

 question with the writer if Cycadeoidca {Raumeria) Masseiana from the "scaly 

 clays" of Italy, C. Uhleri from the Potomac of Man-land, and C. nigra from the 

 Upper Jura (?) of Colorado, are not one and the same species. 



In the case of the upper cycad-bearing horizon of the Black Hills, the main 

 localities of which are at Minnekahta and Black Hawk, South Dakota, certain char- 

 acteristic groupings may be readily made out, while the single trunks from isolated 

 points some miles from these main localities present, as might be expected, 

 certain intermediate general characters of silicification, coloration, compression, 

 erosion, aud density. The Black Hawk series is light-colored and much chalce- 

 donized. It is a striking one in every respect, a fact which is, however, in part clue 

 to the unbranched columnar habit of most of the trunks, although there are some 

 low round trunks, as at Minnekahta, and at least two species are common to both 

 localities. Clearly these differences in general appearance, in large measure due to 

 what might be termed the accidents of preservation rather than to actual difference 

 in structure, have led to some unavoidable duplications of species before the series 

 of trunks was as complete as at present. 



