154 GEOLOGY OP THE HIGH PLATEAUS. 



older Mesozoic members does not prevail in this one, for it is highly varia- 

 ble not only in the mass, but also in the constitution of the beds. In some 

 exposures it is more than a thousand feet thick; in others, it is less than two 

 hundred. Where its volume is greatest it is more arenaceous, and where 

 the volume is less the beds are shaly, marly, and calcareous. Usually sev- 

 eral seams of limestone occur, and in these the fossils are found often 

 abundantly. One notable feature is the small amount of cement in the 

 arenaceous layers, which are, therefore, very poorly consolidated, and the 

 rock weathers and wastes away with extreme facility. Gypsum and sele- 

 nite occur abundantly in these beds, and especially noticeable is the latter 

 mineral, which is seen sparkling and glittering in the sunlight in the bad- 

 lands to which the decay of the strata gives rise., 



THE CRETACEOUS. 



Throuo-hout the District of the Hio-h Plateaus and the broad terraces 

 which flank it upon the south and east the Cretaceous system has the same 

 relative magnitude and importance which distinguish it in other portions 

 of the West. In absolute mass it is inferior only to the Carboniferous ; 

 but as the latter formation is usually covered by later ones over the greater 

 part of the West, and especially of the Plateau Country, the Cretaceous 

 exposures are everywhere the dominant ones and most conspicuous. The 

 series consists of many beds of sandstone and argillaceous shale, the latter 

 decidedly predominating. The number of beds is very great, but they 

 show a tendency to form groups, here a series of sandstones with a few 

 shales, there a series of shales with a few thin seams of sandstone. Two 

 conditions, however, have combined to render the group a difficult one to 

 study and to correlate with coeval groups in other regions. The first is the 

 want of sharp and persistent divisional horizons ; the second is the great 

 variation of the lithological characters along the outcrops, and the changes 

 which almost all the strata undergo as we trace them from place to place. 

 No two sections show any close agreement in the bedding. Since the fos- 

 sils are generally confined to a few of the many layers, it is frequently dif- 

 ficult to find a valid separation, and even when we discover one we cannot 

 apply it to every locality. But while we are often at a loss to decide to 



