INTRODUCTION. 249 



His opinion upon the age of "the great masses of granite observed in 

 western Burnet and all through Llano County," is that 



They are pre-Potsdam — in part cotemporary with the deposition ol the sediments of the 

 Llano group, but more largely the result of extrusions of granite at or near the close of the 

 erosion of the Llano group and before the deposition of the Potsdam. 



In a foot note he adds: 



It may be that further and more complete observations will prove all the granite to have 

 been intrusive in the Llano group prior to its erosion, but from the evidences, as seen by 

 the writer, it is difficult to explain its occurrence except as above. 



Mr. Walcott has more recently concluded, provisionally, that the Llano 

 series is the geologic equivalent of the Grand Canyon and Keweenawan 

 series, all of which he is inclined to regard as of pre-Cambrian horizon.* 



To this idea I believe he still adheres, for in a recent paper read before the 

 American Geological Society f he places the Grand Canyon series as a mem- 

 ber of the Algonkian system, lying between the Archaean group and the Cam- 

 brian system. 



The next expression of opinion upon the geology of the Central region is 

 by an author already much quoted, whose stratigraphic work in adjoining 

 areas is admirable, but who has never penetrated this part of the district far 

 enough to speak from actual observation, except concerning its eastern border 

 in part. Mr. Robt. T. Hill, of the United States, Arkansas, and Texas Sur- 

 veys, late Professor of Geology in the University of Texas, at Austin, has 

 been for years a most indefatigable worker in the field of Cretaceous Geol- 

 ogy, and incidentally he has had occasion to review the work of others in the 

 region we are now discussing. His masterly treatment of the subject in his 

 excellent treatise J is the most concise and satisfactory resume of our knowl- 

 edge up to the date of its publication. Since then we have nothing in print 

 except certain allusions by Mr. Hill himself in later papers, which contain 

 some generalizations, which appear to be more comprehensive than are really 

 warranted by the facts as now revealed. 



In a review of Texas Cretaceous strata§ he says: 



The latter (Paleozoic) are exposed along a north and south axis from the Red River to the 

 Colorado by the removal of the overlying Cretaceous. 



* Second Contribution to the Studies on the Cambrian Faunas of North America. Bulletin 

 United States Geological Survey, No. 30. "Washington, 1886, pp. 57-64. 



f Study of a Line of Displacement in the Grand Canyon. By C. D. Walcott. Bulletin 

 Geological Society of America, vol. I, p. 50. 



X Present Condition of Knowledge of the Geology of Texas. Bulletin United States Geo- 

 logical Survey, No. 45, 1887, pp. 55-7, 87. 



§ The Texas Section of the American Cretaceous. By Robert T. Hill. American Journal 

 of Science, vol. XXXIV, Oct., 1887, p. 301. 



