PALEOZOIC ROCKS. 663 



THE JURA-CRETACEOUS SYSTEM. 



It is not proper for me to speak with much detail concerning the geology 

 of the post-Carboniferous border of the Central Mineral Region. My studies 

 have not extended far enough into that area to give me an adequate knowl- 

 edge of its stratigraphy very much beyond the contacts with the earlier 

 strata. At the same time, it has been necessary to study some features with 

 care in order to interpret the structural and dynamic history of the area more 

 immediately under review. Thus the determination of the age of the latest 

 prominent disturbance in the inner tract required that all the rocks involved 

 in that dynamic action should be seen, at least to a reasonable distance be- 

 yond the border. Having ascertained that a large area of uncovered Silurian 

 rocks lies along the Pedernales River within the Cretaceous area, and being 

 obliged to traverse large tracts of the latter strata in visiting special Paleozoic 

 outcrops and in going to and coming from the field, considerable information 

 was gleaned regarding the later terranes, much of it from districts not pre- 

 viously examined, at least since the date of Roemer's visit. 



The dynamic event recorded in the Central Mineral Region by lines of up- 

 lift, fracture, and faulting, trending north 50° east, extends out into the Jura- 

 Cretaceous border area upon the north, east, and south, and apparently upon 

 the west also, although it seems to me to be less evident in the region of Men- 

 ard, Concho, and western Kimble counties. Whether it includes what Mr. 

 Hill defines as Upper Cretaceous, I can not say, because there are no rem- 

 nants of that terrane within our field. 



In using the term Jura-Cretaceous instead of Lower Cretaceous in this 

 place there is no intention of asserting a belief in any particular chronology, 

 but rather a desire to place the matter exactly where the able workers in these 

 strata have seemed to leave it, i. e., undecided. 



We are dealing here chiefly with the Trinity and Fredericksburg divisions, 

 the former being well exposed upon the east and southeast of the district, but 

 thinning out gradually upon the west until it is wholly absent in certain con- 

 tacts. In most places where the Trinity-Fredericksburg contact is well dis- 

 played the two terranes seem to be somewhat unconformable, but in the 

 absence of very explicit study I am unable to do more than to report this 

 fact as an observation of some possible stratigraphic moment. 



The conclusion announced in 1889 that the Cretaceous sea did not cover 

 the whole of the Central Mineral Region has been strengthened by the obser- 

 vations since made. The evidence is not easily summarized in the space 

 allotted here, but a brief outline is given below. 



First. Over an area of several thousand square miles there are no rem- 

 nants whatever of the Cretaceous rocks, not even in the form of Quaternary 



