THE 



GEOLOGICAL MAGAZINE 



NEW SERIES. DECADE V. VOL. X. 



No. XI.— NOVEMBER, 1913. 



OB-IG-insr^^Jl. .A.I^TIOIliElS. 



I. — Notes on the Semi-Aeid Conditions in a Part of Soittheen 



Texas. 



By T. 0. Boswokth, D.Sc, B.A., F.G.S. 



(PLATES XVI AND XVII.) 



Introduction. — The conditions described below were observed over 

 a large part of South- West Texas ; but the local details apply to an 

 area of one or two hundred square miles in McMullen Count}'', 

 geologically surveyed by the writer in 1912. The centre of 

 observations was a little mining ranch named Crowther, about 

 50 miles south of San Antonio and 80 miles from the Gulf of Mexico, 

 near the inland limit of the Gulf coastal plain. The country for 

 many miles around Crowther is practically untouched by man, though 

 a somewhat primitive little local railroad now under reconstruction 

 reaches to within 16 miles of it. 



Geological Structure of the Region. — The coastal plain is, say, about 

 100 miles wide, extending along the coasts of Louisiana, Texas, and 

 Mexico. The strata are very slightly inclined towards the sea and 

 strike approximately parallel with the coast. Apparently there has 

 been a gradual recession of the sea, accompanied by a slight uptilting 

 of the land, so that the Quaternary and, latest Tertiary were not 

 deposited far inland. Near the coast the plain is nearly flat and 

 almost at sea-level. Inland the ground rises, imperceptibly at first, 

 and then by successive gentle steps. Towards the inland limits of 

 the plain there has been considerable denudation, and the rocks are 

 well exposed. Continuing through this more rocky country, at 

 length we reach the block-faulted Cretaceous beds beyond the plain. 

 In the area considered, the United States Government has not yet 

 carried out either its geological or topographical survey, but 

 bulletins and maps have been published dealing with oue or two 

 other portions of the plain. ^ 



Climatic Conditions. — From east to west along the coastal plain, 

 there is a gradual change in climate, the Louisiana and South-East 

 Texas end being moist, whilst the South- West Texas end is dry. The 

 rainfall in McMullen County is about 22 inches, and most of it falls 



^ C. W. Hayes & W. Kennedy, " Oil Fields of Texas-Louisiana Gulf Coastal 

 Plain " : Bulletin B 212, United States Geological Survey. 



DECADE V. — VOL. X. — NO. XI. ■ 31 



