358 
SILUEIA. 
[Chap. XIV. 
thonia, the calcareous flagstones (Pleta) are underlain by a bituminous and 
inflammable stale (see p. 355), formerly described by General Helmersen *. 
Occurring at Tolks and Pungern, this shale is very rich in well-preserved fossils, 
including many Trilobites of the genera Ampyx and Trinucleus, as well as the 
long-known typical British shell, Leptsena sericea of Sowerby. 
Then follows a fine-grained limestone, which, forming the northern half of 
the Isle of Dago, is, according to Professor Schmidt, who has worked out the 
details of this region, the upper limit of the Lower Silurian rocks of Russia. 
The fossils contained in this band are indeed all truly of the age of the Caradoc 
beds of Britain, including the Calymene brevicapitata, Encrinurus multiseg- 
mentatus, Leptsena sericea, Orthis Actonise, 0. testudinaria, with Murchisonia 
bicincta, Hall, and various types known in rocks of this age in America. 
This limestone, which contains also several characteristic fossils of the inferior 
1 Pleta,' including Orthis lynx and Cheetetes Petropolitana, terminates upwards 
in beds laden with Corals of the genera Halysites, Heliolites, Caninia, Sarci- 
nula, &c. 
Upper Silurian. — In the large work on Russia, Upper Silurian deposits, like 
those of Gothland, were only spoken of by myself and companions as existing 
in the Isles of Oesel and Dago. With the limited time at our command, and in 
a flat country much obscured by Drift, we did not pretend to draw any defined 
line between the Lower and Upper Silurian. Seeing, indeed, that Pentamerus 
oblongus occurred in a calcareous band ranging above the Lower Silurian cliffs 
of Esthonia, it was natural on my part to refer that stratum, as I had done in 
England, rather to the top of the lower than to the base of the upper group. 
Subsequent researches, however, and the preponderance of certain Wenlock 
fossils, have rightly led the Russian geologists to class this Pentamerus-band 
with the Upper Silurian f. In truth, the whole series in Russia, as in Scandi- 
navia, is of such very small vertical dimensions (not, perhaps, a fortieth part of 
the magnitude of the grand British deposits) that, in a region where all these 
strata are conformable, and where, to use the expression of my associate Key- 
* In addition to various able memoirs which bourg, 27 Janvier, 1852). Eepresenting a con- 
he has written on the crystalline and palaeozoic tinuous band of Upper Silurian between the De- 
rocks of Eussia, to the geological maps which he vonian and the Lower Silurian to the west of St. 
has prepared, and to his late works on the De- Petersburg (a tract which I never examined), he 
vonian rocks, my associate in the Imperial Aca- admits that to the south and east of the metro- 
demy of St. Petersburg, General Helmersen, has polis the succession is what my friends and my- 
published an account of the nature and position self represented it to be, viz. Devonian rocks at 
of this shale. It is certainly one of the oldest once resting on Lower Silurian, with one or two 
deposits which have combustible properties ; but minute traces only of the lower band of the Upper 
the petroleum of the Longmynd and of Pitchford Silurian, or the limestone with Pentamerus ob- 
(see p. 27) is of far older date, and in rocks longus. Wherever, as in Eussia and America, 
wherein no trace of land plants has been detected, that fossil, or its representative form, appears for 
It is probable that most of the bituminous and the first time, in ascending order, in strata other- 
anthracitic beds of these early periods owe their wise charged with Wenlock fossils, the geologist 
carbon and hydrogen chiefly to decomposed ma- has a perfect right to class the rock as Upper 
rine vegetables. (See Helmersen, ' Sur le Schiste Silurian. Kutorga had previously shown that the 
argileux-bitumineux d'Esthonie,' Journal des Pentamerus-zone of Esthonia is surmounted, in 
Mines de Eussie, 1838, p. 97.) the Islands of Oesel, Dago, and Mben, by still 
t M. Ozersky, who translated the work by my- higher Silurian strata. These islands seem, in 
self and associates, ' Eussia-in-Europe,' into the fact, to be equivalents of the southern end of 
national language, was the first person to call at- Gothland (Hoburg;, where I have described cer- 
tention to the succession of those upper calcareous tain strata which represent the Ludlow rocks 
Silurian strata, though, when he wrote, the fossil (Quart. Journ. Geol. Soc. Lond. vol. iii. pp. 1 &c). 
distinctions now worked out were unknown. Af- In another work, ' Uebersicht des Silur-Schich- 
terwards, in a short article (Ermann's Archiv fur ten-Systems Lievlands und Esthlands,' 1852, M. 
Eussland, 1848, p. 236), Dr. Pander noticed the su- Schrenk, the well-known explorer of the botany 
perposition of a rock charged withPentamerus ob- and mineral constitution of the North Ural Moun- 
longus and many Corals to the well-known lower tains, has, in conjunction with Professor Schmidt, 
limestone of St. Petersburg. Another important extended considerably the range of the Upper 
contribution to clear up the subdivisions of tlie Silurian ; but the work of the last-mentioned au- 
Eusso-Baltic Silurians was made by Professor thor is that which, in conjunction with the great 
Kutorga, in a map of the Government of St. Pe- palaeontological labours of Pander, has recently 
tersburg, the result often years of labour (Compte thrown the clearest light on the subject. 
Eendu de la Socie'te Mine'ralogique de St.-Pe'ters- 
