10 BULLETIN 77, UNITED STATES NATIONAL MUSELTM. 



formations, although locally other sediments are present. These two 

 predominating members gave rise to Schmidt's divisions Al and A2. 



Al. Blue clay and associated strata. — This division, corresponding 

 to the Lower Cambrian of the general time scale, is composed mainly 

 of plastic, blue to green clay with sandy layers, restuig directly upon 

 pre-Cambrian gneiss and granite. In a well boring near St. Peters- 

 burg, a maximum thicloiess of 300 feet for these clays and associated 

 sandy layers was noted. Interbedded with the clay are sandy layers 

 which, in the lower part of the division, sometimes form a well-defined 

 bed of sandstone. Glauconite grains supposed to be casts of foramini- 

 f era, and doubtful remains of algae, are the only organic remains noted 

 in these lowest beds. Alternating clay and sandstone strata holding 

 large numbers of a very small Ortlioceras-like fossil named Volbor- 

 tTiella, and fragments of stalks and arms, probably of cystids, termed 

 Platysolenites , succeed the more typical unfossiliferous blue clay 

 which forms the middle division of the formation. Other fossils of 

 this horizon are Mesonacis miclcwitzi, two species of Scenella, medusae, 

 and the brachiopod, Miclcwitzia monilifera. The medusae and the 

 last-mentioned species are known from the Lower Cambrian Eophy- 

 ton sandstone of Sweden. This fact, in connection with the occur- 

 rence of Mesonacis, fixes the age of the strata. Thirty to fifty feet of 

 unfossiliferous sandstone succeed tliis Russian equivalent of the 

 Eophyton sandstone, and upon stratigraphic grounds are correlated 

 with the Fucoid sandstone, the next higher member of the Lower 

 Cambrian in Sweden. 



AS. Ungulite sandstone. — The Lower Cambrian is followed by an 

 unconsolidated yellow sandstone quite distinct from the underlying 

 Fucoid sandstone, but bearing a conglomerate composed of fragments 

 of the latter at its base. Phosphatic brachiopods, chiefly Ololus 

 apoUinis, are exceedingly abundant in the upper beds of this division 

 which has received the name of Ungulite sandstone on account of the 

 resemblance of this brachiopod's muscular impression to the mark 

 of a horse's hoof. The change of sedimentation and the basal con- 

 glomerate are indicative of a gap between the two formations, the 

 length of which can not be determined from the Russian section. 

 Comparison with the Swedish section places this Ungulite sand- 

 stone in the Upper Cambrian.^ 



Until the present time, the Cambrian strata of Russia, as well as 

 of other countries, have yielded no bryozoans, but the valves of a 

 number of specimens of Oholus from the Ungulite sandstone were 

 found to be incrusted with a ctenostomatous-like bryozoan described 

 here in later pages as Heteronema priscum. 



1 Prof. Schucliert has called my attention, too late for correction in the text, to the fact that the 

 sandstone holding .0&o?MS apoZZma in Sweden is now referred to the basal Ordovician by the Swedish 

 geologists. If this eorrelation be correct, Heteronema priscum, the oldest-known bryozoan, is of more 

 recent age than Mihsxto supposed. 



