PHYSICAL AND FAUNAL EVOLUTION 49 



To make a distinct system of them, as has been proposed by some, 

 will not solve the difficuhy, because the transitional beds are likely 

 to be of very variable quantitative and chronologic values in different 

 localities. The accepted base of the Ordovicic — the summit of the 

 Saratoga formation in New York, of the Franconia sandstone in the 

 Mississippi Valley, and of the AsapheUus homjrayi beds on the 

 Atlantic coast — is a perfectly satisfactory one, as long as the syn- 

 chroneity of these formations is granted. (Compare Figs, i and 2.) 



REGRESSIONAL PHASE OF THE BEEKMANTOWN 



As has been fully demonstrated by the author elsewhere' and 

 by Berkey,- the chief event of Beekmantown time in North America 

 was the widespread regressive movement of the sea and the re- 

 emergence of the continent. The extent of the movement is shown 

 by the extensive disconformity between the Beekmantown and the 

 succeeding Chazy formations. From this it appears that only a 

 narrow trough remained in the Appalachian region as the sole repre- 

 sentative of the interior or Mississippian sea, while most of the 

 Pacific coast region, west of the Rocky Mountains axis, was prob- 

 ably uncovered (see map. Fig. 3). In the interior of North America 

 the emergence was accompanied by widespread continental deposi- 

 tion recorded in the St. Peter sandstone. The detailed character- 

 istics of this formation; the all but complete absence of fossils; the 

 cross-bedding shown in many exposures; the rounded character of 

 the sand grains, their grooved and pitted surfaces; the absence of 

 the finer impurities; the uniformity of the size of grain in the same 

 region — all point to long-continued shifting about of these sands 

 by winds, and testify against their marine origin. The inclusions 

 in the quartz grains show them to be derived from the crystalline 

 oldland, the chief source being probably the Canadian shield. In 

 some cases the contact with the underlying formations is abrupt and 

 disconformable, showing that erosion of the uncovered limestones 

 preceded the deposition of the sands. Not infrequently the contact 



1 A. W. Grabau, "Physical Characters and History of Some New York Forma- 

 tions," Science, N. S., Vol. XXII., pp. 528 ff., October, 1905; also, "Types of Sedi- 

 mentary Overlap," Bull. Ceol. Soc. Amer., Vol. XVII, pp. 616 ff. 



2 C. P. Berkey, " Paleogeography of St. Peter Time," Bull. Geol. Soc. Amer., 

 Vol. XVII, pp. 229-50. 



