414 :\IESOZ()IC FLORAS OF UNITED STATES. 



Still further evidence is supplied by the fact that in a large number of 

 cases pebbles and coarse sand are firmly cemented to or lodged in tlie 

 alveoli of the trunks. In some instances, however, there is evidence that 

 these pel)l)les and sand grains are of later origin. One trunk, for example 

 (J. H. U., No. 1), exhibits a definite pebbly conglomeratic zone, which 

 does not lie in the plane of compression (see PI. LXXXl). Moreover, the 

 variety in composition of the pebbles of this zone is suggestive of their 

 origin from redeposition in the Pleistocene, as is also the somewhat worn 

 condition of the trunk. The complete or nearly complete trunks range in 

 size from about 28 by 20 by 15 cm. to 49 by 45 by 25 and 50 l»y 42 by 

 13 cm. 



The tissues of all the trunks are replaced l)y silica, and there are 

 occasional coatings of quartz druse. Mineralogically the fo.ssils are 

 pseudomorphs after cycadean trunk tissues. The histology" is evidently 

 not so faithfully preserved as that of the trunks from the Black Hills. 

 Their megascopic characters, both external (PI. XCIX) and internal (PI. 

 XCVI), are, on the contrary, somewhat more satisfactorily shown. 



In hardness the trunks show considerable variation, ranging between 

 5 and 7, apparently due to var,ying porosity. The specific gravity of the 

 hardest and least porous pseudomorphs is about 2, 1 . 



The trimks commonly exhil^it sti-ong compression like those of silici- 

 fied coniferous wood with which they are often associated. The flattening 

 is usually lateral, showing that the trunks were usually em]:)edded in 

 prostrate position. From this fact and the circumstance that all of the 

 sands of the Potomac group are current bedded, one may infer that the 

 trunks were probably transported by water before being entombed. A 

 few are compressed from above downward (PI. XCIV) and a few 

 obliquely (PI. LXXXVH, Fig. ii, 10). 



The cross sections of the complete trunks are therefore usually 

 elliptical in outline (PI. LXXXIII, Fig. 4). Their longitudinal sections 

 range from a well-defined oval (PL XCIX), with the enlargement toward 

 the base, through subrcctangular (PI. CI), to an inverted trapezoid (PI. 

 CVI). The outlines of the complete trunks are suggestive of those of such 

 oljjects as the pineapple, pine cone, sponge, thatched l)eehive of the old 

 pattern, wheat stack, etc., most of which terms, as will later be seen, were 



" Tlip internal stnictiirc iiiul Iiistnn- of tlic .Miirvliind cvciuls will l)p consideicd in a siibsoqueut paper. 



