RAYMOND: CORREL.\TIOX OF THE ORDOVICL\X STRATA. 227 



Lower Ordovician, 



The dra'^'ing of the lower boundary of the Ordovician at the base 

 of the shales with Dictyonema flabclUformc 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 tornquisti), 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 sowth, the base of the Ordo^•i- 

 cian (Obolus sandstone, Ceratopyge shale, and Dictyonema shale) 

 rests on successively higher and higher strata, varying in age from 

 lower Middle Cambrian to upper f pper 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. 



