620 A. W. GRABAU TYPES OF SEDIMENTARY OVERLAP 



division which agrees more fully with the Arbuckle Mountain section. 

 Comparing with this the Mohawk Eiver section, 250 miles to the north, 

 we find a striking discrepancy. In the Mohawk section less than 500 feet 

 of Beekmantown rest with a basal conglomerate upon the Adirondack 

 gneisses, and is followed after an erosion interval by at the most 30 feet 

 of Lowville (= Upper Stones Eiver or Upper Chazy) . This is conform- 

 ably succeeded by the Black Eiver and Trenton limestones. 



It is evident that we have here much the same relationship that exists 

 between the Upper Mississippi region and the Arbuckle Mountain section ; 

 only, in the case of the eastern section, no sands were deposited during 

 the period of retreat, and hence none during the advance. While the 

 Beekmantown of the Mohawk valley is probably not lowest Beekmantown, 

 and although some erosion went on, during the retreat of the sea, in the 

 exposed area, nevertheless it seems not unlikely that at least 1,000 feet of 

 Beekmantown were deposited in central Pennsylvania during the period 

 of retreat, while, if the reference of the "barren" beds to the Chazy is 

 correct, nearly 2,500 feet of limestones were forming in Pennsylvania 

 during the readvance. If the barren beds are Beekmantown, the amount 

 of deposition in Pennsylvania during the readvance would only be about 

 200 feet, which amount agrees more nearly with the rate of deposition 

 shown in the Arbuckle region during the Saint Peter advance. 



The case here set forth takes account only of the greater movements 

 and their results. That there were minor movements is shown by the 

 several sandstones intercalated in the Simpson formation of the Arbuckle 

 mountains and in the Ozark series of Missouri. These, however, did not 

 alter the main course of events to any perceptible degree. 



The Dakota Sandstone problem. — The Dakota sandstone presents 

 another interesting problem of a retreatal sandstone worked over by a' 

 readvancing sea. As already noted, the marine sedimentation at the end 

 of Fredericksburg or the beginning of Washita time extended northward 

 as far as central Colorado. On the Purgatoire river, where the Dakota is 

 100 feet thick, it is underlain by 50 to 100 feet of dark shales and shaly 

 sandstones similar to the Dakota and carrying in the shaly portion 

 Inoceramus comancheanas Cragin. Pholadomya sancti-sabce Roenier. 



Trigonia emoryi Conrad? Protocardia texana Conrad. 



Cardium kansasense Meek. Leptosolen conradi Meek. 



Cyprimeria sp. Tapes sp. 



This rests on 15 to 60 feet of coarse gray cross-bedded sandstone, which 

 in turn rests on the Morrison beds.* Similar conditions exist in 

 Oklahoma and New Mexico. 



* Stanton : Journal of Geology, vol. xiii, 1905, pp. 657-669. 



