556 CENTRAL MINERAL REGION OF TEXAS. 



PART I. 



GENERAL REVIEW OF THE GEOLOGIC 

 STRUCTURE. 



The area of Pre-Carboniferous rocks comprising the Central Mineral Re- 

 gion, as defined in the Report of this Survey for 1889,* was estimated at less 

 than four thousand square miles. The more complete survey of 1890 has 

 materially extended the boundaries of the tract by the discovery of uncov- 

 ered Silurian and Cambrian strata in extensive fields hitherto supposed to be 

 capped by the Cretaceous. The maps accompanying the present Report in- 

 clude only enough of the territory outlying to give a fair idea of the relations 

 of the Pre-Carboniferous terranes to those of more recent origin. These 

 sheets represent an area of six thousand square miles, of which considerably 

 more than five thousand square miles contain exposures of an earlier date 

 than the Carboniferous. 



The preliminary classification of the strata of the Central Mineral Region, 

 as announced in the Report of 1889, requires no important modifications 

 after another season's field study. At least it may be stated that all the posi- 

 tive assertions there made have been confirmed by more extended observa- 

 tions, and none of the provisional announcements have been found erroneous. 



At the same time, more office work is needed upon the collections before 

 the paleontological evidence of the tripartite character of the Cambrian can 

 be said to be clear, and there are still many problems which can only be 

 solved by work of a much more detailed character than has been possible 

 under existing circumstances. In the present Report the economic results 

 must be given special prominence, for which reason the outline of the stratig- 

 raphy here introduced is prepared with the primary object of affording a 

 kind of key to those whose practical needs preclude the task of selecting from 

 the mass of technical description the particular details which apply to indi- 

 vidual cases. Those who desire a more complete account should consult the 

 First Annual Report of this Survey, pp. 339-391. 



There are representatives of the Archean, Eparchean, and Paleozoic groups 

 in the Central Mineral Region, which, as we now understand them, are re- 

 garded as the geologic equivalents of the Laurentian, Ontarian, Algonkian, 

 Cambrian, Silurian, and Devonian systems of other parts of the United States. 

 To avoid difficulties which might arise should our provisional correlations 



*A Preliminary Report on the Geology of the Central Mineral Region of Texas. By Theo. 

 B. Comstock, F. G-. S. A. First Annual Report of Geological Survey of Texas. E. T. Dum- 

 ble, State Geologist, Austin, 1890, p. 239. 



