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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 were 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 Vastergétland at Ekedalen and Skéfde, 
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 
usually to be formed. In any 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 “Orthocera’s limestone” (B,,, B,,,;) 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 B,, and 
~ By, 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 (B,,.) and the Orthoceras limestone (B,,,,), 
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- 
