THE OLDER MESOZOIC OF ARIZONA. 19 



exposed faces do not present sharp angles, but have rounded forms, due 

 m the main to the influence of winds, which wear off the jagged appear- 

 ance but do not tend to form chimneys or assume fantastic shapes. 

 These sandstone ledges, which are very uniform in composition, some- 

 times have a thickness of 100 feet or more, though such heavy beds are 

 usually interrupted by several layers of the shale. 



Toward the lower part of the Moencopie beds the shales gradually 

 become calcareous, and there is in nearly all good exposures a horizon of 

 white, impure limestone, well laminated in its central portion, but becom- 

 ing very thin and hard below and finally passing either into the typical 

 shale or into homogeneous marls. The extreme upper and also the 

 extreme lower portions of the Moencopie beds always. consist, so far as 

 observed, of the typical dark-brown argillaceous shale, and the whole 

 series, wherever the contact can be found, always rests in marked uncon- 

 formity upon the underlying Paleozoic rock (Upper Aubrey). It is very 

 probable that the lower portion of the Moencopie beds belongs to the 

 Permian. 



THE SEINARUMP FORMATION. 



This constitutes a vast assemblage of strata with a maximum observed 

 thickness of at least 1,600 feet. It presents a number of phases, some of 

 which are so distinct that if s|,udied in only one locality they would naturally 

 be regarded as separate subdivisions, but such a general survey as I have 

 been making points to a certain homogeneity in all these beds, or at least 

 establishes the unmistakable tendency toward the recurrence in any of 

 the phases of features that are prominent in other phases. The Shin- 

 arump constitutes the horizon of silicified trunks, and there is no part of 

 it in which fossil wood does not occur in great abundance. It also marks 

 the limit of the wood-bearing deposits of this region. For this reason 

 aloiie, in view of the etymology of the name, I should be justified in 

 extending the Shinarump as far as the fossil trunks occur, and it is obvious 

 from the language used that Major Powell had the upper portions of the 

 formation in view as well as the lower when giving the name, although 

 other geologists, in speaking of the Shinarump, usually seem to have in 

 mind only those beds which he called the Shinarump conglomerate. It 

 is doubtful, however, whether the remainder of the formation has really 

 been studied or carefully observed by others. 



