RAYMOND: CORRELATION OF THE ORDOVICIAN STRATA. 227 
LOWER ORDOVICIAN. 
'The drawing of the lower boundary of the Ordovician at the base 
of the shales with Dictyonema flabelliforme instead of at the top of that 
zone is due to Moberg (73), and seems to have been suggested to him 
in the first instance by the discovery low in the zone of a trilobite 
(Hysterolenus térnquisti), which bore a marked resemblance to Cera- 
topyge. The suggestion was a remarkably good one and is borne out 
both by faunal and stratigraphic relations. 
On the faunal side, one may cite the occurrence in this zone of the 
oldest graptolites, thus marking the introduction of an entirely new 
faunal element. Also, a few species are common to the shales of the 
upper part of the zone and the Shumardia or Ceratopyge shales at 
the base of the Ceratopyge zone. 
The best argument is, however, based upon the evidence which the 
geographical distribution of the deposits show of a great transgression 
of the sea at this time, and the evidence of preceding erosion. This 
evidence has already been detailed above, and it is necessary here only 
to call to mind the conditions in the several regions. 
In Esthonia the Dictyonema shale is interstratified with the Obolus 
sandstone, leaving no doubt that the two are of the same age. The 
Obolus sandstone, with conglomerate at the base in places, rests on 
Lower Cambrian sandstone. 
In Oeland, in passing from north to south, the base of the Ordovi- 
cian (Obolus sandstone, Ceratopyge shale, and Dictyonema shale) 
rests on successively higher and higher strata, varying in age from 
lower Middle Cambrian to upper Upper Cambrian, the Obolus sand- 
stone being developed over the Middle Cambrian Tessini sandstone. 
In Dalecarlia, the Obolus sandstone overlaps onto the crystalline 
rocks, but eastward in Gastrikland boulders of both Lower Cambrian 
sandstone and of Obolus sandstone indicate that conditions there were 
formerly as in Esthonia. 
These facts show very clearly that at the end of the Cambrian there 
was uplift accompanied by some, though probably not great tilting, 
and a considerable amount of erosion, before the deposition of the 
strata of the Dictyonema zone. 
In America, the Dictyonema flabelliforme fauna is known from a 
number of localities in the northeastern part of the United States and 
Canada, but nowhere are these strata found in such position that their 
relation to other strata can be definitely determined. 
