RAYMOND: CORRELATION OF THE ORDOVICIAN STRATA. 193 



if completed as planned, also furnish a complete section through the 

 limestone. This formation is quite thick in the eastern exposure on 

 the Walchow but is very poorly exposed, only the basal portion being 

 cut by the extensive quarries, and the greater part being seen only at 

 places along the river bank just below Dubowiki. 



At the base of the formation one finds the so-called " Lower Linsen- 

 schicht," a rather soft clayey limestone six to twelve inches in thick- 

 ness, full of small flattened grains of about the size, shape, and color 

 of small Leperditias, with which Schmidt first confused them. These 

 small "lentils" have a concretionary form and have been shown on 

 chemical analysis by KuplTer (27), to consist of clay containing iron 

 oxides and calcium phosphate. 



The layer containing these linsen does not seem to have been formed 

 under abnormal chemical conditions, for it is fossilif erous, often highly 

 so, being in fact noted as the best stratum for PUomcra fischcri and 

 Lycophoria nucella, and the fossils are of full size and show no abnor- 

 malities. It is probable that the " Linsenschicht " really represents a 

 basal conglomerate for the Vaginatenkalk, for in places, as at Reval 

 and Packerort, there is a real conglomerate which replaces the "Lower 

 Linsenschicht." 



East from Reval this conglomerate Avas not seen, and west from 

 that city there is no "Lower Linsenschicht," but both the conglomerate 

 and the "Lower Linsenschicht" represent the basal bed of the forma- 

 tion. 



The "linsen" of the Linsenschicht are almost opaque in the thinnest 

 sections which can be obtained of these soft rocks, but show a definite 

 concentric structure. 



An analysis of some of them from Ontika by Kupffer is as follows: 



SiOs 5.93 



AI2O3 3.95 



FesOs 09.92 



MnoOs 26 



MgO 1.21 



CaO 46 



P2O5 1.99 



HO 12.42 



Organic 68 



Residue ' 3.09 



99.91 



