RAYMOND: CORRELATION OF THE ORDOVICIAN STRATA. 187 



phase. The still later deeper water limestone phase, with the Cera- 

 topyge fauna, seems never to have reached this region. 



IMickwitz fully recognized the physical evidences of an erosion 

 period between the Lower Cambrian and the time of deposition of 

 the Obolus sandstone, and in the preface to his paper on Obolus (33), 

 he gives some excellent detailed sections of the Obolus sandstone. 

 He recognized the basal conglomerate at Packerort, which he illus- 

 trated by a diagram, and he gave also a diagrammatic representation 

 of the strongly eroded top of the Lower Cambrian sandstone at Jam- 

 burg on the bank of the Luga, east of Narwa. At this latter locality, 

 the Obolus sandstone, with conglomerate, fills hollows and cracks in 

 the Lower Cambrian sandstone. 



The dark shale at the top of the formation is thicker at Packerort 

 than in any other section, and, as has been noted already by Schmidt, 

 it thins to the eastward, until it is entirely absent at Xama. Still 

 further east it comes in again and is seen far eastward, being four and 

 one half feet thick at Papowka and one foot on the Lawa at Wassil- 

 kowa. Bassler cites the Aariable thickness of the Dictyonema shale 

 as evidence of erosion before the time of deposition of the overlying 

 "Glauconite sandstone." The evidence on this point does not seem 

 entirely clear, and the presence at Narwa of only four inches of " Glau- 

 conite sand" at the point where all the shale is missing does not favor 

 that interpretation, as one would expect the greatest amount of sand 

 in the deepest erosion hollows. Moreover, the glauconite sand is 

 thickest where the shale is thickest, and suggests the alternative 

 explanation, partly borne otit by its fauna, that the "Glauconite 

 sandstone" may really belong to the Packerort formation, representing 

 the deposits of the third and emergent phase of the cycle. It should 

 be noted that the Dictyonema shale is tisually unfossiliferous, fossils 

 being common only along the western portion of its outcrop, the most 

 western locality being on the island Odensholm where they are found 

 in loose pieces cast up on the shore and the most eastern so far re- 

 ported being on the Isenhof stream between Asserien and Ontika.^ 



I Eichwald (17), however, reports Dictyonema from as far east as Zarskoe Selo, south of 

 Petrograd, and also at Narwa, where there is no Dictyonema shale. According to Schmidt 

 (47), Dictyonemas have been found in lenses of hmestone at the latter locality, and this was 

 probably the source of the ones reported by Eichwald. This of course suggests that the Glau- 

 conite sand at Narwa may not be a representative of the real Glauconite sand as developed at 

 other locedities, but a residium from the Dictyonema shale. Lamansky (29, p. 197) states 

 that graptohtes have also been found in the Lower Linsenschicht at Narwa. and that they were 

 sent to Dr. Holm for study, but I have found nothing more in the literature about them. 

 Lamansky thought that they would prove to be the same as those found by Holm in Oeland 

 (Tetragraptus fauna). 



