PETRIFIED FORESTS OF ARIZONA. 299 



existence of another exactly similar stratum at this horizon or that of 

 a fault, or, what would amount to the same thing, a slide or slipping 

 down of a large block of the uppermost beds in such a manner as not 

 to disturb their stratigraphical arrangement. 



The first of these hj^potheses is rendered improbable by the fact that 

 a careful study of the beds at the same horizon in other places revealed 

 no such stratum, and it could scarcely be so local as not to be found 

 elsewhere. The second hypothesis seems everj^ way probable, as in 

 such a much-disturbed region it would be easy for the erosive agencies 

 to undermine a small outlier or mesa and cause it to sink down intact 

 to a lower level. The question, however, requires more detailed inves- 

 tigation than I was able to give to it. 



Leaving this phenomenon out of the account, therefore, and consid- 

 ering the two exposures, in which there is no question as to their nat- 

 ural position, we may use them as a means of determining whether the 

 strata have any dip, and to some extent in ascertaining the amount and 

 direction of the dip. The topographic map has a 250 feet contour 

 interval, which is too large to be employed with any very great accu- 

 racy, and an aneroid can hardly be depended upon for measurements 

 made six hours apart, as had to be done in this case, but as nearly as I 

 could judge from all sources of information the Natural Bridge mesa 

 seems to be between 50 and 100 feet lower than the southwestern mesa. 

 As the distance from the one to the other is about 5 miles, the dip to 

 the northeast is somewhere between 5 and 10 feet to the mile. As, 

 however, the strike was not accurately determined, there is no certainty 

 that this is the true dip of the strata, and more precise observations on 

 a much larger scale will be necessary to settle this question. 



Although there is no longer any question as to the true stratigraph- 

 ical position of these profuse vegetable remains, there are many facts 

 which stand in the way of the supposition that the trees actually grew 

 where we now find them. Several accounts ^ profess that stumps occur 

 erect with their roots in the ground, showing that they grew and were 

 buried and petrified on the spot, but I was unable to confirm any such 

 observations, and on careful inquiry of residents of the country, who 

 had minutely examined every part of the area, I ^as unable to learn 

 of a single indisputable instance of such an occurrence. The only 

 trunk that I saw standing on end was one that was inverted and had 

 its roots high in air I In fact, from the nature of the case, as 1 have 

 just shown, there would be no use looking for any such phenomenon 

 in any of the principal fossil forests, since they all lie from 100 to 400 

 feet below where the}^ were originally deposited. It is only in the 



iTagebuch einer Reise vom Mississippi nach den Kiisten der Siidsee, von Balduin Mollhausen. 

 Leipzig, 1858, p. 300. 



Rt'sume explicatif d'une carte geologique des Etats-Unis et des provinces anglaises de r Am^rique 

 du Nord, etc., par Jules Marcou. Bulletin de la Socioto Geologique de France, 2« st'r.. Vol. XII, 1855, 

 p. 871. Repeated in Geology of North America, etc., Zurich, 1858, p. 13. 



