600 A. W. GRABAU LOWER ORDOVICIC FORMATIONS 



The fauna of this latter horizon in the Baltic Provinces is, according to 

 Lamansky, very distinct, not a single species found in any member of 

 Division B passing upward, while many new genera make their first 

 appearance. 



Physically the two divisions, B II and B III, are separated by a hiatus 

 which shows an increasing magnitude westward (figure 5). This is indi- 

 cated by the progressive failure westward of the upper members of B II 

 and the lower members of B III, until near Baltic Port both series are 

 chiefly represented by their extreme members. B IT at this point shows 

 a much eroded surface and is followed by a conglomerate and sandstone 

 containing fragments of the underlying bed and resting variously on 

 B II a, B II /?, or more rarely on B II y. The sandstone represents 

 B III y, so that B III /? and B III a are wanting through overlap. Evi- 

 dently after the deposition of the three members of B II an eastward 

 retreat of the sea followed, resulting in the laying bare of the deposits 

 which were then eroded. After this they were progressively covered again 

 by the transgressing sea, with the successive deposition of the westward 

 overlapping members of Division B III. Thus B III a fails in the 

 vicinity of Saint Petersburg and B III ft in the neighborhood of Eeval, 

 each in turn being overlapped by the succeeding formation. The Chas- 

 mops or Echinosphau'ites limestone alone is continuous and without 

 marked lithic change over this area. 



Eastward of Baltic Port the base of the upper series (B III y) is a 

 limestone with a basal phosphatic conglomerate formed of fragments of 

 the underlying Megalaspis limestone. At Putilowo the Megalaspis beds 

 still show marked erosion, followed by a glauconitic conglomerate, while 

 beds with Asaplius expansus (B III a) begin to appear. Still farther 

 east the surface of the underlying bed is smooth, and the contact is marked 

 only by glauconite grains in the overlying bed. The glauconite grains 

 above this contact show evidence of attrition, as do also the fossils found 

 in B III a in this region, these being often strongly worn and frequently 

 broken. 



That the hiatus between B II and B III is greater than is indicated 

 by the physical break described is evident from the complete change in 

 fauna, and it is hardly to be questioned that, if proper exposures occurred 

 farther east in Russia, not only would additional beds be found above 

 B II y — that is, below the break — but others below B III a — that is, 

 above the break. I have elsewhere 33 correlated this progressive-trans- 

 o-ressive movement with the one I had established in the North American 



33 Physical and Faunal Evolution of North America, etc. Journ. Oeol. and Outlines of 

 Geological History, edited by Willis & Salisbury, p. 65, 1910, 



