﻿PRESERVATION AND EXTERNAL CHARACTERS. JJ 



cause produces such wide variations in general appearance or makes specific values 

 more difficult to distinguish. It is a matter of common observation that even 

 alter mineralization is complete fossils of any kind are subject, as relatively solid 

 bodies, to more or less plastic transformation. And it may be well, since we are here 

 dealing with rigidly silicified stems, to cite the fact well illustrated by H. Reusch 

 (Fossilfiihrende Krystallinischen Schiefer von Bergen, Leipzig, 1883), that as the 

 result of strong compression the pebbles of certain conglomerates, usually of the 

 hardest and most resistant quartz, have been flattened and ranged in parallel planes. 

 In some instances this process of pebble-flattening has gone so far that the original 

 conglomeratic nature can scarcely be detected. From such facts as these it is readily 

 seen that one might expect that a mineralized fossil trunk which lias not suffered 

 some compression would be the exception. Especially would tin's be true of any 

 plants with a large central body of soft homogeneous tissue like the pith of the 

 cycads, since mineralization doubtless more often proceeds from without, and the 

 pith in its central and protected position might long remain in a partially mineralized 

 and distinctly plastic condition. Nor is it always an easy matter to detect the results 

 of compression, because most trunk-forming plants are subject to certain irregu- 

 larities of growth, so that a region of laterally narrowed xylem cells, for instance, 

 might or might not mean alteration of their original form by compression. Bearing 

 in mind the foregoing remarks, some of the more conspicuous results of compres- 

 sion may now be given. 



I ertically shortened trunks. — These are very frequent, though more difficult to 

 detect, unless there has been much crushing and decay at the summit at the time 

 of fossilizatiou, resulting in the familiar "crow's nest" form. In the splendid 

 branching specimen (Yale cycad No. 300 ; cf. plate xn), with four branches of very 

 large size, the effects of both vertical and oblique pressure may be readily observed. 

 The largest and evidently the parent trunk of the group, marked a in plate xn, is 

 considerably foreshortened, crushing having affected the form and position of the 

 lateral leaf bases somewhat ; while either after some solidifying had taken place or 

 before decay the summit was crushed down into the medulla — the resulting appear- 

 ance being as if a portion of the medullar tissue had been forced upwards in a 

 distinctly hydrated and viscid condition, so as to partly flow out over partially pre- 

 served portions of the crown of leaves. Some of the marked lateral compression 

 of two of the branches, but by no means all, may have been present in life. (See 

 plates xn and xin.J 



Obliquely compressed or sheared trunks. — In a few instances, where leaf bases 

 are seen to droop instead of occupying their normal more or less ascending position, 

 it is permitted to assume that some alteration in position has taken place as the 

 result of a lateral or circumferential shearing pressure. In some cases the entire 

 trunk has been subjected to a shear, with greater or less distortion and crushing, 

 so that the leaf bases of one side droop, while those opposite ascend more sharply 

 than in life. Of course a shearing force might come from either a vertical or a 

 horizontal direction, and hence affect a trunk embedded in either an upright or 

 inclined position in much the same manner. 



