SIGNIFICANCE OF MOENCOPIE SHALES 387 



applied to the Moencopie shales, it involves the following assumptions: 

 First, the sea must have been almost tideless, for there is no evidence of 

 the erosion which the rise and fall of the tides would occasion; second, 

 the sea must have been almost devoid of life ; third, there must have been 

 a remarkable series of earth-movements, whereby at subequal intervals 

 the land was again and again depressed, so that the surface time after 

 time stood at practically the same depth below sealevel. 



The first two assumptions present no special difficulties, although to- 

 day we know of no tideless, lifeless sea of the kind demanded. The third 

 assumption presents greater, although not insuperable, difficulties. It de- 

 mands that a large number of downward movements of the earth's crust 

 should have been of such dimensions that each one depressed the whole 

 of an area of hundreds of square miles to essentially the same depth of 

 a few feet below sealevel; that is, the successive movements always car- 

 ried the surface down just far enough to let it be covered by the sea, but 

 never far enough to let it be submerged more than a score or two of feet. 

 For some reason, the level of the sea determined the amount of movement 

 of the earth's crust, although there seems to be no good reason for the 

 existence of any such relation. 



The hypothesis that the alternating red and white strata preserve the 

 record of a strophic period during which a lake alternately contracted 

 and expanded presents no such difficulties. The Moencopie shales closely 

 resemble the Seyistan clays, and the two may represent corresponding 

 phases of the Permian and Pleistocene strophes. If this is so, the Moen- 

 copie land on its emergence from the Carboniferous sea must have been 

 warped into a flat-floored basin. At first subaerial deposition appar- 

 ently prevailed, but later, on the advent of the changeable conditions 

 of a strophe, lacustrine conditions prevailed during arsial epochs, and the 

 basin floor was exposed to subaerial deposition only during thesial 

 epochs. 



America is not the only part of the world where alternating red 

 and white strata of Permian age are found. In Arabia, Palestine, Sinai, 

 and Kubia there is a thick body of non-fossiliferous strata, chiefly com- 

 posed of red sandstone. This "desert sandstone," as it is called, lies 

 conformably between a thin bed of limestone containing Productus and 

 other Carboniferous fossils and a thick limestone full of Cretaceous 

 fossils. Its lower portion, which occupies the stratigraphic position of 

 the Permian, is said by Lartet to contain layers of shale and marl. Hull 

 says that near mount Sinai "the lower beds are generally white, suc- 

 ceeded by red, and these by yellowish strata. . . . Language almost fails 

 to convey to the reader an idea of the effect produced [at Petra] by the 

 alternations of yellow, orange, red, and purple tints, of varying depths, 



