1857.] MURCHISON — SILURIAN ROCKS OF RUSSIA. 45 



masses which, as formerly shown by my colleagues and myself, con- 

 tain the Devonian shells of Devonshire and the Rhine, united in the 

 same beds with the fishes of the Old Red Sandstone of Scotland. 



However small may be the number of species common to two of the 

 Silurian zones, examples are not wanting to show that, when a change 

 occurs, each typical fossil is replaced in the next succeeding band by 

 an analogous form, which, in its turn, is constant throughout the 

 entire extent of its stratum. Now, as this continuity of forms stops 

 at the uppermost limit of the Silurian rocks, the phsenomenon, as 

 M. Schmidt observes, seems to operate as much in favour of the 

 distinction of the various bands or stages, as it clearly does for their 

 union in one system of life. 



The researches of M. Schmidt have furnished him with nothing 

 new respecting the inferior portion of the Silurian strata, which, pass- 

 ing from the Government of St. Petersburg, underlies the cliffs of 

 Esthonia called Glint. That lower series consists in an ascending 

 order of sands and clay, Ungulite-grit, argillaceous schists, and a 

 chloritic grit and limestone, as described in 'Russia and the Ural 

 Mountains.' The summit of the cliffs is composed of grey, flag-like 

 limestone, the " Pleta" of the Government of St. Petersburg, and 

 marked No. 1 in M. Schmidt's map. This limestone ranges a little 

 southwards into the continent, disappearing at a very gentle angle 

 under the superior formations. The blue clay, cropping out to the 

 east of Reval only, sinks on the west beneath the sea-level. The 

 "Ungulite-grit" contains its usual fossils, Obolus Apollinis, Orbicula 

 Buchii*, and O. reversa ; whilst the schists present us with the 

 Graptopora (Gorgonia) Jlabelliformis, Eichw. (the Phylloyrapsus of 

 Angelin), which, in Sweden, is characteristic of the lowest fossil-beds 

 or alum-slates. 



In the green sandf or chloritic rock of Esthonia, no one has posi- 

 tively discovered the existence of the microscopic tooth-like bodies that 

 M. Pander has collected from the same bed in the Government of 

 St. Petersburg, and described as fish-teeth. Along with the Obolus, 

 however, M. Schmidt has detected a Lingula and the Orthis calli- 

 gramma. The chloritic limestone is specially developed at the foot 

 of the cliffs of the Isle of Odinsholm. Grains of chlorite are there 

 strikingly disseminated in a greyish-white limestone. This rock 

 constitutes a marked feature, and is equally visible at Baltisch Port 

 and Reval. In point of organic remains, it contains numerous frag- 

 ments of Asapki, and notably of a species resembling A. Tyrannus, 

 Murch. (the A. heros of Dalman). This chloritic limestone passes 

 into the Government of St. Petersburg without containing a peculiar 

 fauna. 



The calcareous flagstone called Pleta, No. 1, is exhibited in great 



* The names of all the fossils in this part of the memoir are those given by 

 Prof. Schmidt. The species have not been yet compared, like those of Norway, 

 with their British analogues. — R. I. M. 



f Prof. Ehrenberg's researches (Berlin Transact. 1855) tend to prove that a 

 portion at least of the green grains in this sand has been derived from the minute 

 stony casts of the shells of Polythalamia. 



