NORTH AMERICA AND THEIR VERTEBRATE FAUNA. 



19 



Udden,* in 1912, presented a very full account of the rocks and other 

 deposits of Archer and Wichita Counties, most of which would be included 

 in the Wichita formation as it is conceived by the author: 



"It is believed that these sections (see p. 34) represent different parts of a 

 general section some 300 feet thick. They are known to include successive strata 

 measuring about 225 feet, and constituting the uppermost beds exposed in the 

 field examined. We shall regard these sections first as a group representative of 

 a single formation. They all occur within the areal limits of what has come to be 

 known as the Wichita formation." ^ 



"The thickest section measures only 130 feet in all, and most of them fall 

 short of 50 feet. The total number of feet of rock described is only a few feet 

 more than 1,000. Some items are known to be repeated descriptions of the same 

 strata, in places where the sections are not very far apart, and many more of them 

 must include beds that are synchronous and were made at the same time. 



From these sections we may obtain a fairly close estimate of the gross nature 

 of the formation. As seen in the exposures, it consists of shales, sandstones, con- 

 glomerates and limestones, named in order of their rank as to bulk. Seventy-nine 

 per cent of the total thickness described in the section consists of shale, 20 per 

 cent of sandstone, and less than i per cent each of conglomerate and limestone. 

 There are also gradations between all of these groups. It is believed that the 

 percentage of sandstones determined in this manner is higher than the actual 

 percentage of sandstone in the formation, as this rock stands weathering better 

 than the shales, and is hence more frequently preserved in the outcrops. But 

 the difference between the actual and apparent ratios of sandstone and shale can 

 not be very great. In section 25 [section 34 of this paper] above, where there is a 

 total of 124 feet of shale and sand, the percentage of shale is 83, and that of sand- 

 stone 17. The relative quantities of different rocks are shown in the following 

 table, in which the several rocks described and measured in the sections are classi- 

 fied, summed up, and reduced to percentages : 



' " Table showing total thicknesses in feel and percentages of different kinds 

 of rocks described in sections seen in Wichita and Clay Counties. 



AVERAGE THICKNESS OF DIFFERENT BEDS. 



"The beds described vary in thickness from less than i foot to 60 feet. Not 

 one of the conglomerates or limestones is more than 3 feet thick, and only 11 in- 

 stances were noted of sandstones exceeding 5 feet. Of these only 3 were more than 



» Udden, Bull. University of Texas No. 246, 1912. 



*> W. F. Cummins, First Annual Report Geological Survey Texas, 1889, p. 186; C. H. Gordon, George 

 H. Girty, and David White, Jour. Geol., vol. 19, igii, pp. 110-134, and others. 



