R. S. Tarr — Topographic Feature* of Texas. «K)7 



Briefly outlined, geological history, commencing as far back 

 as I have studied it. begins with an old pre-Sub-carboniferoue 



land of Silurian and older rocks. At present tins system ap- 

 peal- as a patch of very much metamorphosed rock surrounded 

 <>n three Bides by Cretaceous. Little is known of this region. 

 h is plainly a very much degraded mountainous land-area 

 which has furnished thousands of feet of strata for the later 

 formations. A- to its former boundaries little can he said ; hut 

 I have given elsewhere 4 * what T believe to he evidence of a 

 former northward extension. The tract contains marble, sand- 

 stones and schists of Lower Silurian and earlier age; hut 

 the Upper Silurian and Devonian if they exist at all are mere 

 remnants. The stratigraphy is greatly complicated by faults 

 and fold-. 



In Lower Carboniferous time a submergence and long con- 

 tinued period of sedimentation permitted the deposition of an 

 extensive Beries of Lower Carboniferous limestone, several 

 thousand feet of which are still exposed between the Silurian 

 on the south and the overlapping LTpper Carboniferous to the 

 north. The old shore line of the Lower Carboniferous sea may 

 be plainly traced in favorable places, and here, particularly in 

 the upper beds, may be seen the varied shore deposits of that 

 time. 



A gap of unknown value, accompanied by a tilting and 

 slight crumpling of the Lower Carboniferous limestone series 

 intervened between the close of that period and the beginning 

 of the Upper Carboniferous. The latter beds rest un confor- 

 mably upon the lower limestone. This series fully 8,000 feet 

 thick consists of the usual alternation of beds so typical of that 

 period. f The upper beds indicate an approach to the Permian 

 condition of an inland sea. What intervened betw r een the 

 Permian and Lower Cretaceous has not yet been made out. 

 That a portion of the Carboniferous had a dry land condition 

 for at least a part of the time is proved by the irregularity of 

 the contour beneath the Cretaceous. It is impossible to say 

 to what extent this laud condition was shared by the surround- 

 ing areas ; but the Carboniferous of the Colorado river valley 

 and the neighboring Silurian shows signs of erosion by the posi- 

 tion of the lowest bed of the Cretaceous, the Trinity sands of 

 Hill. The Trinity sands rest on the Carboniferous on the east 

 low as 1250 feet above the present sea level ; but in the 

 western portion they are on the 2000 foot contour line. There 

 is a rise of 750 feet in less than fifty miles. A carefully made 

 map of these beds may reveal to us an interesting history of 

 the immediate pre-Cretaceous land. The Silurian area was 



*The American Geologist, forthcoming number. 

 fSee 1st Annual Report Texas Geol. Survey. 



