294 CENTRAL MINERAL REGION OF TEXAS. 



ing the whole area as late as the close of the Carboniferous period. Previous 

 observers have looked upon the district as an eroded tract exposing the 

 Paleozoic Beds only by the denudation of a thick mass of Cretaceous strata. 

 The uncovered Pre- Silurian rocks, by whatever name heretofore known, 

 have always been regarded as relics of this character. Mr. Walcott, who 

 was able in his short stay to determine the great unconformity at the base of 

 the Paleozoic, and to satisfy himself that some of the granites were Pre- 

 Cambrian, left the subject in about the same condition. Mr. Hill, although 

 pointing out the probability that later granites exist, which have introduced 

 irregularities into the structure, has been strongly imbued with the idea that 

 the Cretaceous strata crossed and covered the region, and that, as stated 

 above, denudation has chiefly made the topography what it now is. 



The facts which have led the writer to adopt a contrary opinion regarding 

 the Pre-Silurian periods have already been given. The Silurian System, in 

 its lithology, and to some extent in its stratigraphy, presents new issues. 

 On account of subsequent events it is not so easy to make out what may 

 have been its shore lines while depositing; yet, when we get a chance to 

 study its contacts with the underlying strata, and to compare its texture, its 

 distribution, and its dynamic history with those of the Cambrian, the idea of 

 a Silurian land area within the district is confirmed. Two facts which bear 

 most strongly upon this question are the following: 



(1) The inauguration of the Silurian period by an epoch of shallow water 

 sedimentation, especially westward. (2) The deposition of later Silurian 

 beds unconformably upon eroded Cambrian strata. Items in point will be 

 given as we proceed. Neither of these important stratigraphic elements has 

 heretofore been reported. 



Close correlation with the Silurian strata of other regions is not now possi- 

 ble, hence, as before, provisional local names are used to designate the sets 

 of beds comprised in this system in Central Texas. The lithologic and pale- 

 ontologic transition from Cambrian to Silurian is not violent; at the same 

 time there is an evident stratigraphic unconformity in most sections. The 

 Silurian has not been studied with the care which has been bestowed upon the 

 earlier rocks, as it has required much labor simply to block out the area 

 covered by the system. Again, the purely structural determinations and the 

 tracing of contacts with later strata have consumed much time. This work 

 is not yet completed, but enough is known to report that the area hachured 

 with horizontal lines upon the progress map is very largely covered by the 

 Silurian rocks. The final map will show an inner border of Cambrian rocks 

 of irregular shape, much broken by downthrow faults with Silurian now at 

 surface. But this belt now occupies only a small portion of the pre-Carbonifer- 

 ous Paleozoic area of our district. A rough classification of the Silurian Beds 



