' RELATIVE AGE OF DETROIT RIVER SERIES 77 



and Grabau, 9 the Amherstburg dolomite can not very well be considered 

 younger than Helderbergian. But since the basal Onondaga at Amherstburg 

 does not carry the oldest fauna belonging to that formation, there is still 

 room for doubt that the arenaceous contact is that representing the Oriskany 

 farther to the east in Ontario and New York. 



As is well known and as already pointed out above, the Oriskany sandstone 

 itself rests unconformably on the Silurian (Cobbleskill and Rondout), at the 

 eastern end of Lake Erie. It may be that the unconformity (disconformity) 

 at the base of the Oriskany in western New York represents the one occurring 

 at the base of the Detroit River series, and that the unconformity (discon- 

 formity) at the top of the Oriskany is the one so prominent at the base of 

 the Onondaga in Ohio, Michigan, and extreme southwestern Ontario. If this 

 be the case, these two unconformities run together at various places in Ontario 

 near the eastern end of Lake Erie, but diverge rapidly to the westward from 

 Springvale. There is thus represented an unknown interval during which it 

 is probable the Detroit River series was deposited ; for to maintain that such 

 a fauna as that found in the Amherstburg dolomite is Silurian is more im- 

 possible than to find a place for it among the recognized Devonian formations. 

 While the fauna of the Detroit River series is not altogether unlike that of 

 the Helderbergian, it differs so markedly from it that it can hardly have lived 

 in that sea which spread westward over New York and even into Illinois 

 during Lower Devonian. The occurrence of some of these same fossils in the 

 dolomites of Alberta, as shown by a small collection from the headwaters of 

 the north fork of the Saskatchewan River, 10 suggests that there may have 

 been another embayment from the north or northwest during early Devonian 

 with a very different fauna from that found in the Helderbergian of the At- 

 lantic embayment, although in part contemporaneous with it. The fact that a 

 fauna similar to that of the Detroit River series, especially in its Mollusca, 

 and to a less extent in the Anthozoa and Brachiopoda, occurs in the lowest 

 Devonian of the Ural Mountain region of Russia 11 lends some support to this 

 suggestion. Even in the Idiostroma limestone of the Cedar Valley of Iowa 

 there is a suggestion of the recurrence, at a much later date, of the coral and 

 hydrozoan reefs of the Anderdon. 



Presented in abstract extemporaneously. 



Discussion 



Dr. A. C. Lane: Over a large area there is no trouble in drawing the line 

 between Devonian and Silurian, as there is such an unconformity that the 

 Onondaga (Dundee) comes almost directly, separated only by the Oriskany 

 sandstone from the Salina or Lower Monroe. But near Detroit River we find 

 that, while there remains an unconformity, or rather disconformity, under the 

 Dundee, the Oriskany seems also to be almost or quite continuous as a trans- 

 gression sandstone with the Sylvania sandstone. If we turn to Schuchert's 



9 Mich. Geol. and Biol. Survey, Pub. 2, Geol. ser. 1, 1909 (1910), p. 46. 



10 A. W. Grabau and W. H. Sherzer : Mich. Geol. and Biol. Survey, Pub. 2, Geol. ser. 1, 

 1909 (1910), pp. 102 and 116. 



ii Th. Tschernyschew : Memoires du Comite" Geologique, vols, iii, 1885, and Iv, 1893. 

 Von N. Lebedew : Idem., vol. xvii, 1902, tables opp. p. 132. 



