﻿26 INTRODUCTORY. 



RARE OCCURRENCE OF CONDITIONS REQUISITE FOR MINERALIZATION OF CYCADEAN TRUNKS. 



That no cycacl trunks have been found thus far in the Upper Cretaceous and 

 Tertiary is noteworthy, for not only are the 107 enumerated living species widely 

 distributed, but the genus Cycas represents an ancient type, doubtless as abundant 

 in the Tertiary as at present. From the evidence at hand we must hence conclude 

 that all of the conditions adduced as necessary to silicification, calcification, or ferri- 

 zatiou of cycads have seldom existed concurrently, and that mainly because these 

 plants were so abundant over all the land areas of the globe during the middle 

 Mesozoic do we find them sparsely represented in strata of that period. It would 

 seem as if cycads were then present wherever conditions chanced to make possible 

 their preservation, and that later not only were these plants less abundant, but that 

 conditions requisite to their preservation or that of similar plants seldom occurred. 

 And it must be likewise regarded as a fact altogether eloquent of the many vicissi- 

 tudes attending the preservation of the structural details of these trunks and their 

 fruits, that almost the only other plant remains ever found accompanying them are 

 the trunks of conifers with scarcely a remnant of their branches, leaves, or fructifi- 

 cations left behind. Evidently the peculiar structure, oils, and resins of the cycads 

 make their preservation mechanically and chemically possible ; and were it not for 

 the protected position in the armor of old leaf bases of the young leaves and mature 

 or nearly mature fruits, both ovulate and staininate, as surrounded by and immersed 

 in the luxuriant growth of ramentum, favoring siliceous infiltration and preservation 

 in minute detail, it is very doubtful if any exact knowledge of their organization 

 would ever have been gained. The general fact is that not only are the individual 

 differences in such specimens very great, but the exigencies of preservation equally 

 so ; for while the abundance of the imprints of cycadean fronds in nearly all plant- 

 bearing strata of Jurassic age, wherever found on the globe, indicates that cycads 

 dominated the vegetation of that period, trunks with structures preserved are rela- 

 tively exceedingly few in number. In speaking of this subject elsewhere (191) the 

 writer has said : 



"Notwithstanding their wide distribution in latitude and time, our knowledge of 

 the ancestry of the living cycads has hitherto been slight, because the fossil remains 

 have, with few exceptions, consisted only in imprints of isolated leaves and fruits. 

 Silicified or calcified trunks with their microscopic structure in any degree preserved 

 have always been among the rarest of fossils. It is, too, a singular fact, showing 

 how precarious have been the chances for the complete preservation of these plants in 

 the fossil form, that all but a very few of the trunks known are distributed between 

 the middle Jurassic and the lowermost Cretaceous. Moreover, though the various 

 cycadaceous forms which have existed since the Triassic may include many families, 

 the trunks known represent only a single offshoot from the main cycadean line, as at 

 present understood, but withal marked \>y very unusual characters." 



TRUNK FORMS DUE TO COMPRESSION DURING OR AFTER FOSSILIZATION. 



The external appearance of fossil cycad trunks, as one immediately notes in 

 the case of any considerable number of specimens, and, as has already been men- 

 tioned, is usually much dependent upon the inequalities of compression to which 

 they have been subjected during or after fossilization. In fact, scarcely any other 



