February 26, 1909] 



SCIENCE 



353 



of the Mohawkian and Cincinnatian are 

 essentially a unit. 



In the Mohawk Valley less than five hun- 

 dred feet of Beekmantown occur, but in the 

 southern Appalachians, from central Penn- 

 sylvania south, its thickness is 2,500 feet or 

 more. This, as has been shown by myself 

 and worked out in detail by Berkey,* repre- 

 sents a regressional movement of the sea, 

 begun in Lower Beekmantown time and con- 

 tinued throughout; so that the full measure 

 of the depositional series is found only in the 

 non-emerged areas of the Appalachian trough. 

 Northward and westward from this the higher 

 beds fail progressively, the series in central 

 United States being complicated by the desert 

 sands now constituting the St. Peter sand- 

 stone. Though it began before the Upper 

 Cambric transgressional movement was fully 

 spent, the greater part of Beekmantown sedi- 

 mentation was accomplished during a period 

 of emergence of the North American conti- 

 nent. 



The Chazy, represented only by its higher 

 beds, the Lowville, in the Mohawk Valley, is 

 nearly 2,500 feet thick in central Pennsyl- 

 vania, and has a similar thickness in the 

 Appalachians southward. From the region of 

 its greatest thickness, which is also the region 

 of maximum development of the Beekman- 

 town, the Chazy thins northward and west- 

 ward by failure of its lower members and 

 progressive overlap of its higher. Except in 

 the region of non-emergence, i. e., the Ap- 

 palachian trough from central Pennsylvania 

 southward, it rests disconformably upon the 

 Beekmantown, the hiatus between the two con- 

 stantly increasing northward and westward. 

 When present, the St. Peter sandstone marks 

 the plane of this disconformity, though its 

 thickness is no measure of the magnitude of 

 the hiatus. The Chazy thus represents a 

 transgressional series, pronounced in its oc- 

 currence over most of North America, but of 

 limited extent in the type region in the St. 

 Lawrence embayment. 



» Science, N. S., Vol. XXII., pp. 528-535, Oc- 

 tober 27, 1905, and Bull. Geol. Soc. Am., Vol. 17, 

 p. 616, 1906. 



* Bull. Geol. Soc. Am., Vol. 17, pp. 229-250, 1906. 



The Black Eiver and Trenton limestones 

 mark the continuance of the transgressive 

 movement. The maximum thickness of the 

 Trenton is less than a thousand feet, and in 

 most places in the east it is partly replaced 

 by the Utica shales. Thus in western New 

 York the limestone referred toTrenton is over 

 950 feet thick, and the succeeding shales are 

 probably Lorraine with little Utica. Eastward 

 the Utica or black shale phase begins earlier, 

 the higher limestones being replaced by the 

 lower shales. Thus in the type region near 

 Utica the limestone is in the neighborhood of 

 300 feet thick, and the Utica shale is YOO feet. 

 Still further east, near Amsterdam, the lime- 

 stone has become reduced to 37 feet, while the 

 shale has increased to over a thousand feet. 

 Northward the limestone increases again to 

 nearly 600 feet in the Montreal region, while 

 the thickness of the shale diminishes. In cen- 

 tral Pennsylvania about 700 feet of limestone 

 and 600 feet of shale occur, but in southern 

 Pennsylvania Stose has found that the Tren- 

 ton is almost entirely replaced by the Utica 

 phase of shale, of which there are almost 1,000 

 feet of strata. These shales are succeeded by 

 the Eden formation, which in the Cincinnati 

 region rests on Trenton limestone. It thus 

 appears that Trenton limestone and Utica 

 shale represent different lithic and correspond- 

 ing faunal phases of the same period of sedi- 

 mentation. 



During the succeeding Lower Cincinnati 

 period an extensive retreatal, followed by a 

 readvance movement of the interior North 

 American sea, occurred. As a result, in the 

 marginal portions of this sea, as in the Eocky 

 Mountains, late Richmond rests on early 

 Trenton or earlier beds. In the Appalachian 

 region, extensive continental deposits marked 

 the closing stages of this period, while in the 

 Taconic and Acadian regions, folding was in 

 progress. 



As already intimated, three faunas are 

 found in the American Ordovicic: (1) that 

 of the Beekmantown; (2) that of the Chazy; 



(3) that of the remainder of the series. The 

 Black Eiver fauna is in a measure transi- 



