EAKLY PALEOZOIC BEYOZOA OF THE BALTIC PROVINCES. 11 



ORDOVICIAN AND EARLY SILURIAN STRATA. 



Very fossiliferous limestones and shales compose the greater portion 

 of the Russian Ordovician. The lowest strata are of dark colored 

 bituminous shales or plastic clays formerly regarded as of Cambrian 

 age, but, with later evidence, now assigned by some authors to the 

 lowest Ordovician. Following these beds is a glauconitic sandstone 

 and then come the more calcareous strata of Middle and Upper 

 Ordovician and Early Silurian age. 



A3. Dictyonema beds. — The uppermost strataof Schmidt's Cambrian 

 division A are dark brown to reddish bituminous shales and clay, 

 named for their characteristic fossil Dictyonema flabelliforme. These 

 strata, which are thin — indeed often absent — are correlated with 

 similar graptolite beds in Scandinavia, England, and elsewhere, and 

 are probably the only equivalent in Russia of the American Beekman- 

 town. Bryozoans are unknown in these strata. 



Bl. Glauconite sandstone. — Resting upon the eroded Dictyonema 

 shale, or upon the lower Ungulite sandstone, are loose, more or less 

 unconsolidated green sands of variable thickness, although seldom 

 exceeding 10 feet. These strata have been considered as the base of 

 the Ordovician by von Schmidt, and in the latest edition of Kayser's 

 ''Lehrbuch der Geologic, II, Geologische Formationskunde," the 

 Dictyonema shale and the Glauconite sandstone are correlated with 

 the American Beekmantown, The unconformable position of the 

 Glauconite sandstone, and the presence of Middle Ordovician fossils 

 in its upper portion, are indicative that it is, in part at least, the initial 

 deposit of the succeeding limestones and shales. 



A very detailed account of the strata and faunas of formation B has 

 been published by Lamansky ^ who recognizes two subdivisions in Bl, 

 namely, a lower subdivision (Bla), with the phosphatic brachiopods, 

 Oholus siluricus and 0. lingulseformis , reminders of Cambrian time, 

 as the characteristic fossils, and an upper subdivision (Bib) holding 

 species of Megalaspis and of Orthis. These Cambrian-like brachiopods 

 are probably persistent forms with little stratigraphic value, but the 

 fauna occurring in the upper part of the greensand where the sand 

 grains are associated with dolomitic material and clay, is composed 

 of typical Middle Ordovician fossils. These species are listed in the 

 accompanying table, where it may be noted that brachiopods and 

 trilobites make up practically the entire list. Comparing this with 

 the preceding and succeeding groups of species, it becomes evident 

 that this is clearly not a derivative of the underlying Cambrian fauna. 

 The general association is much like the succeeding Ordovician faunas, 

 even though entire classes, such as the echinoderms, bryozoans, etc., 

 are unrepresented. Gradually and without break the Glauconite 



1 Die Aeltesten Silurischen Schichten Russlands (Etage B); Mem. Comite Geol., new ser., vol. 20, 1905. 

 92602°— Bull. 77—11 3 



