318 PROCEEDINGS OF THE ACADEMY OF [1893. 



Cretaceous overlap, which is everywhere, on the Carboniferous, a 

 gently undulating line, with no sharply defined buried topographic 

 features. It was a plain and probably a base-leveled plain. The 

 older Palaeozoic core formed the highlands of the region, and here, 

 in consequence of the harder nature of the rocks and their more 

 variable character with reference to hardness and altitude, the 

 topography was distinctly more diversified. 



(cZ) Cretaceous Subsidence. — With the beginning of the Cretaceous 

 period there came an extensive and rapid 9 subsidence, during which 

 both the upper and the lower Cretaceous beds were deposited. The 

 history of this period has been so fully worked out by Prof. Hill and 

 so clearly described by him in various places that I shall do no 

 more than state the fact. This part of the history of Texas is already 

 known to geologists. The several thousand feet of Cretaceous com- 

 pletely buried the older Palaeozoic land as first pointed out by Prof. 

 Hill. 10 This fact has been disputed by Dr. Comstock" who has spent 

 several years in a study of the older Palaeozoic rocks of the Central 

 Denuded Area. Prof. Hill reiterates his statement later and is sup- 

 ported in the conclusion by Mr. Walcott 12 aud Dr. Curtice. 13 I have 

 studied the problem from a standpoint different from that of the 

 geologists above referred to and am convinced of the accuracy of 

 Prof. Hill's deductions, and, in the paper on this subject above referred 

 to, have given my reasons for this conclusion, these being both 

 geographic and geologic. Since they do not fall within the scope 

 planned for this paper I make no further reference to them here. 



This complete transformation of a large area of nearly base- 

 leveled land to a condition of deep sea sedimentation lasted until mid 

 Cretaceous time when there was an elevation followed by another 

 depression in which the upper Cretaceous beds were formed. There 

 was at this time, that is during the upper Cretaceous epoch, a period 

 of volcanic eruption in the vicinity of and southwest of Austin, the 

 relics of which remain now as low necks of nepheline basalt. 

 These, the Shumard Knobs, are, according to Hill, u marine vol- 

 canoes in the Cretaceous Sea. 



9 For proofs of the rapid subsidence I refer to an article by the author in the 

 American Geologist, Vol. IX, 1892, pp. 169-178. 



10 Am. J. Sci.. XXXVII, 1889, p. 283. 



11 First Ann. Rept., Texas Geol. Survey, 1889, pp. 315-318. 

 Second Ann. Rept., Texas Geol. Survey, 1891), pp. 663-664. 



12 Buil. Geol. Soc. Am., 1890, Vol. II., pp. 522-3. 



1 3 Bull. Geol. Soc. Am.. 1890, Vol., II., pp. 526-8. 

 1 Am. ( leol. Nov. 1890, pp. 286-292. 



