2 GEOLOGY. 



succeed occasional exposures of a coarse-grained sandstone, No. 1. Still farther inland we meet 

 with a form of soft calcareous earthy rock outcropping along the sides of hills, and constituting 

 the first outlayer of that extensive cretaceous formation which characterizes so large a scope of 

 country throughout middle and northwestern Texas. 



From San Antonio, occupying the first step in the cretaceous series, at an elevation of 600 

 feet above the Gulf of Mexico, proceeding northerly on the line of the lower road to El Paso, 

 we soon remark a rapid change in the general features of the country. The underlying lime- 

 stone formation becomes more largely developed, and is less deeply covered with alluvial 

 deposits. The rock stratum is frequently exposed in the beds of streams, which are everywhere 

 thickly strewn with the* water-worn pebbles of this formation. The streams here acquire an 

 intermittent character, subject to sudden overflow and recession. Their course, when low, is 

 marked by an irregular series of deep basin ponds connected together only by shallow brooks, 

 or even not at all above ground during dry seasons. 



As we proceed, mural exposures of limestone rock become more frequent, and the same for- 

 mation is met with on summits of the higher ta.ble-land. The alluvial tracts along the course 

 of the larger valleys acquire a more arid character of soil, and support a stunted timber growth, 

 in which mezquite makes its appearance. Fossil, fresh water, and land shells are quite abun- 

 dantly scattered over the lower depressions of these alluvial bottoms. 



At the crossing of the Eio Frio, near Fort Inge, occurs the first exposure of igneous rock. It 

 is seen as an isolated knoll of dark-colored trap, showing at this place but slight disturbance of 

 the adjacent cretaceous strata. This formation is thence observed to constitute a broken line, 

 extending in a northwest course, and coming into view at several points along the road at 

 variable distances of 5 to 10 miles. 



On approaching the line of the great table-land formation of Northwest Texas, we find near 

 its base the sources of most of the minor streams of this region. These sources frequently 

 exhibit magnificent basin springs, of which that at San Felipe is a noted example. 



"We have here reached the main development of this extensive cretaceous formation, partially 

 concealed from view towards the coast, as above noticed, by alluvial deposits, but here standing 

 out in bold relief, variously exposed in extensive ridges, bounding, more or less closely, valleys 



Character of valley denudations in tlie cretaceous tableland formation. Upper portion of San Pedro river, Teias. 



of denudation, or else stretching in vast upland plateaus, thinly covered with soil, and support- 

 ing a close even growth of upland grasses or scanty shrubs. 



