RAYMOND: CORRELATION OF THE ORDOVICIAN STRATA. 229 



The absence of the Lower Didymograptus fauna in Esthonia seems 

 explainable on the basis of lack of suitable physical conditions. It is 

 well known that the abundance of graptolites increases in proportion 

 to the degree of fineness of grain and amount of carbonaceous material 

 in shale. The Middle and Upper Cambrian strata of Sweden are 

 vast storehouses of very fine grained, highly carbonaceous shale. 

 Possibly these deposits extended at one time nearly or quite to 

 Esthonia. As has been shown, the end of the Cambrian was a 

 time of considerable denudation, and the Cambrian sediments could 

 furnish a vast supply of black mud, which, on account of its fineness, 

 could be transported long distances. Hence the widespread deposit 

 of Dictyonema shales. The shales however, rapidly covered the 

 sinking land, and w-ere in turn covered, over large areas, by the Cera- 

 topyge limestone, so that, when the Lower Didymograptus fauna 

 occupied this region, only limited areas of Cambrian strata, such as 

 the island already mentioned in Vastergotland at Ekedalen and Skofde, 

 were subject to erosion. There may have been a small rather general 

 uplift at this time, indicated in Sweden by the change from limestone 

 to shale sedimentation, and in Esthonia by the glauconite sand. To 

 consider the Dictyonema and Lower Didymograptus black shales as 

 reworked Cambrian shales seems more plausible than to think of them 

 as due to certain peculiar conditions under which black shales seem 

 usuallv to be formed. In anv event, Esthonia was at this time out- 

 side the territory which could be supplied with reworked upper 

 Cambrian muds, while sands were immediately available and the 

 graptolite fauna did not reach the region. 



To correlate the Russian "Orthoceras limestone" (B,i, Bm) with 

 any formation in America on the basis of graptolites is rather com- 

 plicated but it can, I think, be done fairly satisfactorily. In the first 

 place there is general agreement, on the evidence 'of numerous species 

 of trilobites, cephalopods, and brachiopods common to both, that the 

 zones from the Planilimbata limestone to the top of the Gigas lime- 

 stone in Sweden and Norway are the equivalent of the zones Bu and 

 Bji, in Russia. As to the exact correlation of the subdivisions there 

 is not so great unanimity of opinion, but as to the bounding formations, 

 the Planilimbata limestone and the Gigas limestone on one side; and 

 the Glauconite limestone (Bna) and the Orthoceras limestone (Bm^), 

 on the other, there can be no question. 



In Sweden the position of this limestone in respect to the graptolite 

 succession is definitely fixed. We know that the Planilimbata lime- 

 stone succeeds the Ceratopyge zone, and that the Tetragraptus phyllo- 



