660 CENTRAL MINERAL REGION OF TEXAS. 



and summit are very irregular, as regards the horizons involved, and the 

 faults are very numerous and confusing. It will require very close study to 

 finally settle knotty questions of this character. 



Dr. Cooper Curtice, in the discussion of a paper upon a different subject 

 read by Prof. R. T. Hill before the American Geological Society, December 

 30, 1890, refers to contacts of the Potsdam sandstones with granites, but his 

 observations being very limited in time and area, he did not see the numer- 

 ous other contacts in which the Cambrian strata of different horizons are in 

 smooth contact with granites, over which they dip steeply in conformity to 

 the "set" of the granite. Previous observers have usually discussed "the 

 granite" of this region, as if whatever conclusion might be drawn from any 

 one exposure, or one class of exposures, must necessarily apply to all the 

 granitic masses of the whole district. In the Report for 1889 I endeavored 

 to make very clear the fact that there are various granites of different ages, 

 and that discussions based upon anything but intimate knowledge of the en- 

 tire Central Mineral Region are profitless and misleading. The Geologic 

 Map, when published, will show this diversity in a striking manner. There 

 are pre- Cambrian granites, but there are also those of later origin, and the 

 complications resulting from their commingling through successive uplifts 

 are not such as can be worked out in a brief period. This Survey has made 

 only a beginning of the study as yet, although it has done many times the 

 work of any previous organization or individual. 



THE SILURIAN SYSTEM. 



But little can be added here to what was announced in 1889 concerning 

 the subdivisions of the Silurian. The field work of 1890 gives no new divi- 

 sions, but confirms those previously announced. Very much interesting ma- 

 terial was gathered, however, with reference to the distribution of the Silu- 

 rian System as a whole, it being found that large uncovered tracts lie beyond 

 the limits previously defined, besides a very considerable area along the Pe- 

 dernales River, in Blanco and Gillespie counties, from which the Cretaceous 

 strata have apparently been removed by erosion. These hitherto unreported 

 tracts present chiefly the Hoover and Wyo beds at surface, but there is also 

 an expression of the Deep Creek Series in the southeast in Burnet County, 

 south of the Colorado River. 



THE DEVONIAN SYSTEM. 



The debatable fossils, some of which I have previously noted as Devonian, 

 some which have been claimed as Carboniferous by other workers, and per- 

 haps a few which Mr. R. S. Tarr regards as sub-Carboniferous, have been 



