1893.] NATURAL SCIENCES OF PHILADELPHIA. 327 



Nearly coincident with this line are a series of volcanic knobs, 

 the Shumard knobs of Hill, and referred by him 22 to marine vol- 

 canoes in the Cretaceous sea. It is not impossible that there exists 

 here an old Cretaceous fault line, a line of weakness which in the 

 Tertiary elevations furnished a slipping plain from which the pres- 

 ent fault of the Balcones resulted. The Shumard knobs at present 

 exist as a series of low, well-rounded hills rising out of the prairie, 

 and made up of nepheline basalt. 



The Grand Prairie proper consists of a series of nearly horizontal, 

 gently east-dipping, alternating hard and soft beds of lower Creta- 

 ceous. Its eastern margin is regular, and coincides with the western 

 margin of the Black Prairie. On the western edge, on the other 

 hand, the escarpment is one of extreme irregularity and weird 

 beauty. The Colorado River and its tributaries have here cut 

 through the Cretaceous and reached the underlying Palaeozoic rocks ; 

 and the young drainage, intensified by the sub-humid conditions of 

 rainfall, has caused a marvellous complication of buttes, mesas, 

 irregular parks, and projecting promontories. The young drainage 

 has cut down to the base of the Cretaceous, but subrenal denudation 

 has not been able to keep pace with the down-cutting, and flat-top- 

 ped divides remain between the streams. A hard stratum of lime- 

 stone caps these, buttes and mesas and is largely responsible for the 

 type of topography. This irregular escarpment almost completely 

 surrounds the region, so aptly called the Butte or Denuded region 

 by Professor Hill, and far away from the scarp, out on the Carbon- 

 iferous strata, isolated buttes, remnants of the former Cretaceous 

 covering, still remain. 



(b) The Cross Timbers. — This Grand Prairie region is treeless 

 except for two narrow strips, the Cross Timbers, which extend 

 approximately north and south across the prairie. Along the 

 streams and in the rougher parts of the area live-oaks and other 

 Texas trees grow in small groves or in isolated patches. The Cross 

 Timbers early attracted the attention of the settlers and many spec- 

 ulations have been indulged in to account for their existence. 

 Loughridge, for instance, in the Tenth Census report states that 

 these occupy two old abandoned river beds. Hill has, however, 

 shown 23 that this is not so, for the timber areas occupy two sandy 

 strips which are not river beds but merely the outcrop of sandy 

 •layers in the Cretaceous. 



il iun. Geo]., Nov. 1891, pp. 286-294. 

 *Um. J. Sci., April, 1887. 



