THE LOWER OR COMANCHE SERIES. 129 



and the including limestone horizons, but the "Washita fauna again ap- 

 pears somewhat modified in the upper portion. At Austin, at the contact 

 of these clays with the Shoal Creek limestone, is found the last horizon of 

 Gryphcea pitcheri, Mort., which appears as the variety navia, but varying in 

 shape and size, being characterized here chiefly by the thickness and size of 

 its shell. 



There are occasional segregations of the fossils into limestone, but these 

 have no persistent extent or size. There are also numerous crystals of selen- 

 ite, which are a product of the reaction of the decomposing iron sulphides 

 (pyrites) upon the numerous oyster shells. These clay beds are worthy of 

 closer study and definition than it has been possible to give them. Like the 

 underlying Washita limestone, they have great geographic extent and uni- 

 formity. Their purity, extent, and apparent freedom from littoral debris 

 make them easily distinguishable. The Arietina clays produce a black waxy 

 residual soil, the only truly black soil of the Comanche series, the others be- 

 ing chocolate black or other dull colors. I have seen them only in Travis 

 and Williamson counties. 



THE SHOAL CREEK LIMESTONE. 



In the western portion of the city of Austin, forming the rocky walls of 

 the Shoal Creek Canyon, and for a few miles north and south, the uppermost 

 strata of the Comanche series consist of beds of a peculiar crumbly limestone. 

 This limestone is from forty to eighty feet thick, and of a yellow color, with 

 many spots of red and pink. 



It is stratified, and upon close examination it is seen to be made up of mi- 

 nute fragments of shell, which are rapidly losing their integrity by alteration 

 either into a harder condition or by breaking down into a pulverulent powder, 

 as in the case of the Caprina limestone before described. The red blotches 

 have been attributed to several causes, to-wit: (1) The decomposition of iron 

 pyrites; (2) the oxidation by heating of adjacent igneous material; and (3) 

 the decomposition of contained volcanic ash and cinder which were deposited 

 contemporaneously with it. This point has not been finally determined, 

 however. 



In places the Shoal Creek limestone is decomposing and crumbling, while 

 everywhere it is much jointed and faulted. The fossils contained therein 

 are interesting, but have been as yet but little studied. The top surface of 

 this limestone has been corroded and waterworn, and deposited unconform- 

 ably upon it can be seen the radically different sub-littoral unconsolidated 

 clays of the basal Upper series. 



This limestone makes a convenient building stone, but of no great com- 

 mercial value. Its residual soil, like that of the Caprina limestone, is of a 



