184 bulletin: museum of comparative zoology. 



Cambrian and Ordovician were lettered in ascending order A to F, 

 and many of the subdivisions designated by numbers as, Ci, C2, C3 and 

 finally some of these subdivisions were further divided by Lamansky, 

 as for instance B,,„, Bn^ and B,j^. The strata also received names 

 suggested by their lithological characteristics or faunal contents, as 

 for instance, Bi was also known as the " Glauconite sand" and Bm as 

 the "Orthoceras limestone" or " Vaginatenkalk." The divisions 

 from A to €1,3 are to be found in the escarpment, and these together 

 received the collective name of the "Glint" but no formation in this 

 part of the series has received a separate geographic name. 



The strata above Ci^s are exposed as has already been explained, 

 principally in quarries, and therefore each formation has received 

 a name from the locality which has furnished either the best expo- 

 sures or the best fossils. Thus C2 is known as the Kuckers, E as the 

 Wesenberg, etc. 



In this particular case, both the system of lettering and the system 

 of descriptive names is objectionable, and for the sake of uniformity 

 the writer suggests a set of geographic names for the older formations 

 of the series. The system of lettering fails, because A2 and A3 of 

 Schmidt prove not to be Cambrian but Ordovician, thus splitting A 

 between tw^o great systems. There is likewise a difficulty about D2 

 which will appear later. A mixed table of descriptive names, part 

 derived from lithological and part from faunal characteristics is never 

 satisfactory, and in this case the names seem particularly inapplicable. 

 Thus the "Orthoceras" limestone is not by any means the only forma- 

 tion in Russia in which Orthoceras is abundant, and the term has not 

 the same meaning here as when applied to Ordovician strata in Sweden 

 or Norway. Likewise Ci is called the " Echinosphaerites " limestone, 

 though Echinosphaerites is equally common at some localities in C2, 

 C3 and the lower part of Di. 



Description of Formations. 

 Cambrian. 



Esthonia formation. Ai and part of A2 (Blauer Thon and lower part 

 of Ungulitensand) of Schmidt. Lower Cambrian. 



Since it has been so well described by Schmidt (42), Mickwitz (33), 

 and Holm (20), the writer paid comparatively little attention to the 

 study of the Cambrian, but examined the exposures at Reval, Port 



