RAYMOND: CORRELATIOX OF THE ORDOVICIAX STRATA. 185 



Kunda, Ontika, Peuthof, and Papowka, and studied especially the 

 contact of the Cambrian and Ordovician near Baltishport and at 

 Xarwa. The best section seen was at the cliff Peuthof, which is 

 north of the station Waiwara, a few miles west of Xarwa. Alternating 

 strata of light colored sandstone and blue clay-shale were there well 

 exposed, but the strata can be studied more in detail at Port Kunda, 

 where Mickwitz found specimens of Schmidtiellus mickwitzi, the 

 raesonacid which first afforded definite proof of the Lower Cambrian 

 age of these strata. 



R has been repeatedly stated that the "Blue Clay" underlies the 

 sandstone of the Cambrian, but I did not find this to be the case. 

 Everv-^v'here the highest layer of the Cambrian appeared to be a hard, 

 usually almost white, sandstone. The upper bed, where its thickness 

 could be seen^ was usually not over fifteen to twenty-five feet thick, 

 and beneath it was a bed of blue clay-shale of variable thickness. 

 Below this again one finds sandstone and alternations of shale and 

 sandstone continue to the base of the cliff, and, according to borings 

 in Reval and Petrograd, su(i)i alternations continue downward about 

 600 feet to the gneiss. The fossils have been found in the upper zones, 

 within fifty feet of the top of the formation, and there is no reason to 

 believe that strata of any age other than Lower Cambrian are present 

 in this formation. The "Blue Clay" of the Lower Cambrian has 

 received considerable notoriety, as it has often been reported as a soft, 

 unconsolidated blue clay which could not be distinguished from clay 

 of glacial age. Masses of this sort were seen at two places, at Papowka 

 south of Petrograd, and on the shore at Ontika. In neither case was 

 the clay actually in position either under or between layers of sand- 

 stone, but it lay in such a position that it could be readily conceived 

 that it was Cambrian clay which had worked out from a layer nearby. 

 In both cases it was very full of water, and it is probable that it 

 represented a portion of a stratum of shale which had been worked 

 up by the action of frost, water, and a creeping movement, until all 

 traces of the original stratification had been destroyed. Where mined 

 from the layers for the cement plant at Port Kunda, the clay is well 

 stratified, and hard. It is, however, very fine grained, soapy to the 

 touch, and a very fine plastic clay. The quickness with which it loses 

 its stratification on weathering is probably due to its fine grain and 

 the readiness with which it takes up water, rather than to the fact 

 that it has never been consolidated. 



In discussing the finds of "Olenellus" at Kunda and near Reval by 

 Mickwitz, Marcou (31) proposed for the Lower Cambrian strata as 



