618 A. W. GRABAU TYPES OF SEDIMENT AEY OVERLAP 



ably succeeded by the Black river. This and the fossils found in them 

 show these beds to be the highest Stones Eiver (Chazy) and equivalent to 

 the Lowville or Birdseye of New York. In the Champlain valley the 

 Beekmantown is at least 1,800 feet thick,* while the Chazy is nearly 900 

 feet thick on Valcour island, f the lowest beds not being shown. 



It thus appears that the Saint Peter sandstone of Minnesota fills the 

 interval represented in the lake Champlain region by the deposition of 

 over 1,500 feet of Beekmantown dolomites and more than 800 feet of 

 Chazy limestones. Its perfect conformity with the overlying and under- 

 lying beds proves that this great hiatus lies within the sandstone itself. 



In eastern Tennessee the Knox dolomite has a thickness of about 4,000 

 feet, of which the upper half, if not more, is basal Ordovicic. It is 

 succeeded by the Maclurea limestone (Chazy, with Maclurea magna), 

 which has a maximum thickness of 600 feet, and this is followed by 

 several hunderd feet of upper Stones Eiver. In central Tennessee the 

 Stones Eiver group is 360 feet thick, at Cincinnati 500 feet, and in central 

 Kentucky about 375 feet. In all these cases it is underlain by a repre- 

 sentative of the Saint Peter sandstone.); 



In the Arbuckle mountains of Indian Territory the upper 1,250 feet 

 or more of the Arbuckle limestone are of the age of the Beekmantown of 

 New York — that is, basal Ordovicic. A slight erosion interval and some 

 beds of pure sand separate this formation from the overlying Simpson 

 series, which has a maximum thickness of 2,000 feet. It includes at 

 least one heavy bed of sandstone near the center. The fauna of the lower 

 half of the formation (below the sandstone) is that of the Chazy of 

 New York, while that of the upper half is similar to the fauna of the 

 upper Stones Eiver of central Tennessee or of the Stones Eiver beds 

 lying between the Saint Peter and Black Eiver, in the Minnesota area. 

 It thus becomes clear that in the Arbuckle Mountain area as well as in 

 eastern Tennessee the whole or nearly the whole of the basal Ordovicic 

 (Beekmantown) was deposited while the sea retreated from the Lake 

 Superior region. That the Arbuckle region was also laid bare toward the 

 end of this retreat is shown by the erosion plane between the Arbuckle 

 and Simpson formations. During the readvance of the sea the lower 

 Chazy beds were laid down in the Arbuckle region, but, as before noted, 

 they thin away northward. This thinning away of basal beds continued 

 throughout the period of advance, until only the upper 32 feet of Chazy 

 (Upper Stones Eiver) were deposited in the Minnesota region. Thus the 



* Brainard and Seeley : Bull. Am. Mus. Nat. Hist., vol. iii, 1890, pp. 2, 3. 

 t Brainard and Seeley : Ibid., vol. viii, pp. 305-315. Bull. Geol. Soc. Am., vol. 11, 

 1891, pp. 293-300. 



t Winchell and Ulrich : Loc. cit., p. xclv. 



