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RECONNOISSANCE SOIL SURVEY OF SOUTH TEXAS. 23 nearly similar that identical soils have resulted. These soils occur almost entirely north of the divide between the Nueces River and the Rio Grande, although a few small areas usually too small to be shown on the map occur south of this ridge. Differences in the percentage of sand in the underlying clays, combined with the influence of the wind in removing the finer particles from the surface material, as well as drifting it in from more sandy formations, have given rise to variations in the texture of the soil, so that two types in the Houston series have been recognized-a loam and a clay loam. The two are usually very closely associated, and a detailed separation was impracticable. Therefore areas shown on the map as one type often represent simply the predominance of that kind of soil. While these soils have been included in the Houston series it is recognized that the drier climate here has caused them to be more calcareous and to contain less humus than is usually the case in the corresponding members mapped in areas farther east. The native vegetation also partakes more of the character of that of the arid region, the larger percentage of cactus and mesquite being very noticeable. However, as these soils represent the southwestward extension of the black prairie region and soon grade into the typical soils of this belt north of here, it has seemed best to include them in this series. HOUSTON LOAM. Description.--The Houston loam consists of a light to dark brown or dark-gray loam, varying in depth from 8 to 18 inches. The dry condition of the soil has permitted the removal of the finer material from the immediate surface, leaving the sand, thus causing the texture sometimes to approach a fine sandy loam. This is especially true immediately adjacent to the divide in Duval County, which is capped by sandstones. -Here the sand from the weathering of these rocks, as well as from other sandy areas, has given the soil a lighter texture. This is more or less the case wherever the type adjoins more sandy soils. The lighter texture is usually found upon the summits of the low hills or ridges, where the percentage of sand may become sufficient to approach a heavy fine sandy loam. The soil contains considerable humus, which gives it a rather dark color and makes it friable and easily cultivated. The color is somewhat darker in the eastern than in the western part of the area, and the surface is often covered by a thin crust, which causes it to have an apparently much lighter color. This darkening from organic matter extends to an average depth of about 10 inches, where there is a gradual change into the subsoil.
