TOPOGKAPHIC EXPRESSION OF THE CAFRINA LIMP:STONE. 513 



by a unique fauna. The typical occurrence of the Comanche Peak horizon 

 is along the sides of the buttes and mesas of central Texas which are capped 

 by ih.Q Caprina limestone, such as Comanche peak and others. The bed is 

 usually covered with a growth of rather thin, scrubby oaks ; the soil is thin 

 or absent, and the angular fragments of the weathering rocks make up the 

 surface. Frequently, however, there are large areas over which the Comanche 

 Peak horizon extends as the surface formation. 



The Caprina Limestone. — The next member of the Comanche Peak group 

 is the Caprina limestone of Shumard. This is the direct continuation of the 

 Comanche Peak chalk, only the limestone is harder and more persistent, 

 and the fossils less numerous and characterized by the occurrence of a few 

 peculiar forms, especially Rudistes, which have already been referred to bv 

 me in other writings. Genetically it is inseparable from the underlying and 

 overlying beds, since there is no sharp demarkation between them. It is a 

 deposit of deeper waters than the underlying Comanche Peak chalk, how- 

 ever, as shown by its lack of lamination and stratification planes. 



At Comanche peak the limestone is between thirty and forty feet thick, 

 and though it increases to the southward it does not change greatly. It can 

 correctly be called an indurated chalk. It is more or less stratified, although 

 usually a great massive bed from top to bottom. Some parts are harder than 

 others, and so make up a curved outline to the bluffs; others are materially 

 softer, and frequently are eroded away, leaving either honey-combed cavities 

 or shelves under the overhanging harder layers. 



Topographically, the Caprina limestone is one of the most important fac- 

 tors in Texas, since its superior hardness and resistance have preserved it as 

 the capstone of the innumerable buttes, mesas and plateaus of the central 

 portion of the state, where it forms a great plane of resistance to denudation. 

 So perfectly does this limestone find expression in the topography that its 

 extent can readily be traced by the highest contours of the United States 

 Geological Survey topographic sheets of Coryell, Bell and other counties. 

 It nAy be said to be the determining factor in the topography of the region. 

 All of the buttes or so-called mountains north of the Colorado are capped 

 by it ; the great scarps which often run for miles overlooking the prairies to 

 the west represent the same stratum ; the walls of the canons which many of 

 the streams have cut are almost invariably composed of the Caprina lime- 

 stone. 



But little need be said in regard to the distribution of the Caprina lime- 

 stone. In northwestern Texas the Double mountains of Stonewall county 

 are capped by the Caprina; so are also Comanche peak, the mesa and almost 

 all the buttes east of the Brazos opposite Glen Kose, the high bluffs marking 

 the canon of the Brazos from the Bluff mills near Kimball far down the 

 river, the buttes and mesas about Walnut and Iredell and toward the south, 



