LOWER SILURIAN ROCKS OF ESTHONIA. 
33* 
Red Sandstone of Scotland, but also from showing the existence of many placoid 
fishes of the highest organization in strata of such remote age. It is, indeed, the 
more necessary to dwell upon this last-mentioned feature, since the Upper Silurian 
rocks of the Scandinavian and Russian Isles, though teeming with other marine 
remains, have not afforded a trace of fishes, and that the ichthyolites here men- 
tioned lie in the very lowest beds of the Devonian rocks of the northern continent 1 
(see Observation and Postscript at the end of this chapter). 
Lower Silurian Rocks of Esthonia . — The chief masses of “pleta” limestone 
which occur along the Baltic coast, where they equally repose upon the Ungulite 
grit and sandstone, and usually with the separation of bituminous schist, as in the 
country already described, offer few mineral distinctions from masses of the same 
age in the government of St. Petersburgh. Near Jeive, a post-station to the west 
of Narva, and between that place and Waiwara, the limestone occupying the sum- 
mit of cliffs about 150 feet in height, has a thickness of from thirty to forty feet, 
and being in many parts completely denuded, exposes a floor of thick, calcareous 
flagstone, absolutely loaded with Orthoceratites, among which the O. vaginatus and 
O. duplex are by far the most abundant, one other species, which is rare, being the 
0. bacillus (Eichw.). 
Like the Silurian rocks of England, these calcareous masses are affected by sym- 
metrical, vertical or highly inclined joints, which, cutting through the horizontal 
strata, divide them into rude prisms. The chief directions of these joints trend 
obliquely to the Gulf of Finland, one set to the north-east and the other to the 
north-west, the line perpendicular to the re-entering angle being from north and 
by west, to south and by east, or, in other words, at right angles to the general 
strike of the formations. The result of this crystalline and prismatic division of 
the heavy calcareous roof of a cliff, the lower parts of which are composed of inco- 
herent materials, is the rapid wearing away of its seaward face ; which presents 
a succession of salient and re-entering angles, like those seen in quarries among 
jointed rocks. The spectator who places himself in the innermost extremity of 
one of these angles has on either side of him a beautiful vertical section of strata- 
1 The presence of very peculiar and small ichthyolites (one of which is the Old Red or Devonian genus 
Onchus) in the Upper Silurian rocks of England was first pointed out hv Mr. Murchison (Sil. Syst. , 
PP- 198, 605). As yet no fossil fishes have been observed in the Upper Silurian rocks of the continent 
of Europe or in America, and in no country has a truce of than been heard of in the Lower Silurian . 
F 2 
