32 MESOZOIC FLORAS OF UNITED STATES. 



were transported from Lithodendron Creek by an expedition headed by 

 Lieut. J. T. C. Hegewald in the spring of 1879." The only species that has 

 yet been described from the silicified wood of Arizona is the Araucarioxylon 

 arizonicum of Knowlton, based on specimens from these two trunks. 

 Neither of these trunks is colored, but both of them show structure. The 

 importance of these specimens, therefore, and of the locality at which 

 they were found will be readily understood. 



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

 I was imperfectly 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 I believed at the time that the coarse gravels in 

 which I found the logs in place really l^elonged to it. I was, however, 

 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 occurrence 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 waj^ in Avhich these IdccIs decline to the eastward. The 

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

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

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

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

 petrified wood, is sufficient 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 practicall}^ 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 being laid down in the position in 

 which the}' occur. 



«See bis report in Proc. U. S. Xnt. Mus., Vol. V, 1SS2, pp. 1-3. 



