TRIASSIC. 507 



being said to have the greatest analogy with the Gryplisea dilatata of the Oxford clay." It is 

 probably a new species or a variety of Gryphsea pitcheri, and a description of it will be found in 

 Chapter IX, under the name G. tucumcarii, originally proposed by IVIr. Marcou. 



From these various observations we may conclude that the great plateau, the Llano 

 Estacado, consists of horizontal strata of light-colored white or grayish-white and yellowish 

 sandstones and bluish clays and marls, resting conformably upon beds of red clay, red sand- 

 stone, and gypsum, and these, according to Mr. Marcou, are intercalated with grayish or light- 

 colored sandstones, as on Kocky Delaware Creek. It also appears that the light-colored sand- 

 stones and calcareous beds constitute the main part of the bluffs and that the red marls and 

 clay of the gypseous series are at their base and occupy the space between them and the river. 



The strata assigned to the Jurassic by Marcou were by Cummins ^^' shown to 

 be Lower Cretaceous. The Triassic age of the lower part of the section is neither 

 proved nor disproved. The strata beneath the Cretaceous may be Triassic, or they 

 may be older red beds (Permian or Pennsylvanian) from which the Triassic beds 

 were eroded. 



In western Texas Cummins distinguished and Drake ^"^ described the Dockum 

 group, of Triassic age, a sequence of calcareous sandstone, clay, cross-bedded 

 sandstone, and conglomerate, deposited on an eroded surface of Permian. Drake 

 recognized three divisions, which he described in detail. 



The Dockum beds underlie aU or nearly aU of the Staked Plains of Texas and southeastern 

 New Mexico, extend farther back into New Mexico northwest of the plains, and have some 

 extension under the Cretaceous area so.uth of them in Texas. 



The limit of the plains on the east, north, and west is marked by an escarpment which is 

 usually from 100 to 200 and sometimes 300 or 400 feet high. The basal portion, sometimes 

 nearly all of this escarpment, is composed of the Triassic beds. These beds usually extend 

 6 or 7 miles from the base of the escarpment and nearly surround the plains by a narrow band. 



Sandstones, conglomerates, and clays constitute nearly all of the strata of this formation. 

 The materials composing the difTerent strata vary somewhat in lithological characteristics 

 at different localities and even at the same locality, but the general characteristics are quite uni- 

 form and are so different from the underlying Permian and overlying Cretaceous or Tertiary that 

 they are usually easily recognized. This is especially true of the sandstones and conglomerates. 



Sandstones. — The sandstones before exposure to weather are generally nearly white, but 

 sometimes gray, red, or bluish in color. Massive, shaly, and false bedding are common. The 

 texture varies from a fine, even-grained to a grit or conglomeratic sandstone. White and a few 

 brown mica flakes, varying in size from a mere speck to one-eighth of an inch in diameter, 

 are nearly always present. This mica is so abundant in some of the rocks as to make them 

 fissile. The sandstones are usually friable but weather with a smooth, flat surface and with an 

 average sharpness of angle for sandstone rocks. 



Conglomerates. — ^The conglomerates are of two kinds. The one most characteristic and wide- 

 spread is composed of small pieces of brownish, yeUowish, or bluish colored subangular 

 indurated clayey sandstone fragments, averaging about the size of a pea, embedded in a matrix 

 of sand or grit, usually calcareous. The other is composed of sihceous pebbles in a matrix of 

 sand and grit. The pebbles are usuaUy small and well rounded and of nearly all shades of color, 

 but wliite quartz are the most numerous. The quantity of siliceous pebbles varies at different 

 localities from more than half the rock mass to very few. Both conglomerates contain silicified 

 wood at some localities. The bedding of the sihceous conglomerate is usually even and regular 

 or shghtly false, while that of the first named is false almost Avithout exception. These two 

 conglomerates graduate into each other, and even where one is the most characteristic the other 

 usually enters into it more or less. 



" The genus Gryphsea was formerly included with the Ostracea. 



