BITUMINOUS SCHIST AND “ PLETA” LIMESTONE. 
28* 
Bituminous Schist . — This schist, sometimes of darkish green, but usually of 
black colour, which occurs pretty generally between the Ungulite grit and the 
limestone (and was classed with the former by Strangways), is mineralogically not 
unlike many beds known in the carboniferous rocks of England. It may be com- 
pared with the “ bat” of the Staffordshire coal-field, and might even be assimilated 
to certain hard beds of the Kimmeridge clay, so little do lithological characters 
alone enable us to decide upon the age of rocks. It is specially distinguished by 
containing rounded, or oblate sphseroidal nodules, or calcareous concretions, the 
surfaces of which are marked by projecting crystals of calcareous spar and pyrites 
radiating from a common centre. Being highly carbonaceous at the falls of the 
Sablenka, a tributary of the Tosna, it is there partially extracted from beneath the 
limestone, for the manufacture of coarse pencils. Though this black schist is very 
persistent and is seen in all the sections of the hills which range fromDuderhof to 
Czarskoe-celo, as well as on the banks of the Volkof and Siass rivers, it is of incon- 
siderable thickness, and seldom contains organic remains. In Esthonia, however, 
where it is in parts highly bituminous, a few graptolites and organic remains have 
been discovered in it. 
3. “ Pleta,” or Orthoceratite Limestone.-— Occasionally, as on the Pulkovka 
brook, beyond the observatory, and again on thePopofka,a feeder of the Slavenka, 
some of the bottom layers of the limestone are of a dullish red colour, and have the 
glistening fracture of a sandy dolomite, but in general the lowest strata are cha- 
racterized by containing a profusion of grains of dark green mineral like chlorite 
in a light grey-coloured base. According to the examination of M. Abich of 
Dorpat, it u T ould appear that such green grains are the detritus of the ancient 
augitic rocks of the Finnish frontier; and our friend M. Worth has even detected 
small portions of malachite and “ blei glanz ” in these lower beds. In lithological 
aspect, however, these lower beds usually so resemble the “ craie chloritee” of the 
French, and some varieties of our upper greensand, that when mineral characters 
were the chief guides of geologists, they were even supposed to belong to the cre- 
taceous system ! They are usually separated by way-boards of reddish and green- 
ish-grey shale. These are again surmounted by a considerable thickness of dingy 
grey, earthy, flat-bedded and slightly consolidated limestone. The lower as well 
as the central or argillaceous beds of the limestone, may be well seen in the ravines 
of the St. Petersburg!! hills, extending from Czarskoe-celo to the hills of Duderhof, 
