32 IMESOZOIC FLORAS OF rM'll.D STATES. 



were transported fi'oin Lithotlendron ("reek by an expedition headed by 

 Lieut. -I. T. ('. Hegewald in the spring of 1879. " 'J"he only species that has 

 vet l^een described from t lie silicified wood of Arizona is the , 1 niurnri().vijl()ii 

 nrizonicum of Knowlton, based on specimens from these two li'unks. 

 Neither of these trunks is colored, but l)otli of them show structure. The 

 importance of these specimens, therefore, and of tlie locality at wliicli 

 they were found will be readily understood. 



At the time I made the investigation upon which my report was leased 

 I was imperfecth- acquainted with the geological relations of the forma- 

 tion in general, as set forth above, and I treated the subject from the 

 narrower standpoint of such a knowledge of the immediate region of the 

 petrified forests as I was able to acquire in the short time devoted to their 

 study. I did not in my report even so much as mention the Shinarump 

 conglomerate, although 1 believed at the time that the coarse gra^'els in 

 W'hich I fovmd the logs in place really belonged to it. I was, howe\-er, 

 mistaken in supposing that there was only one bed of this conglomerate 

 and that the rocks forming the summit of the mesa on which the Natural 

 Bridge is situated were the same as those observed on the southwest side 

 of the general area. The last-mentioned beds dip rapidly to the north- 

 east and come down within 100 or 200 feet of the bottom of the wash 

 which passes through the Lower Forest. The occun-ence of fossil wood 

 in place in a very low position a few miles north of this point, which I was 

 somewhat disposed to attribute to faulting, is the perfectly natural result 

 of the regular way in which these beds decline to the eastward. The 

 mesas in the northern part of the forests, including that of the Natural 

 Bridge, have at their summits an entirely different series of conglomer- 

 ates, occupying a much higher position in the general system. This suc- 

 cession of several beds of conglomerate one above another, all filletl with 

 petrified wood, is sufFi(uent to account for the vast quantities that have 

 accumulated since the breaking down of these cliffs and the washing 

 away of the intervening marls, so that the necessity for a theory of 

 extensive transportation is practically removed. It is probable, however, 

 from the considerations set forth in my report, that most or all of the 

 logs were drifted some distance before lieing laid down in the position in 

 which they occur. 



"Soe his rci)ort in Pioc. U. S. Xat. Mils., \\A. V, ISSli, pp. 1-3. 



