44 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 6, 



Grey limestone with shales. Fishes. (No. 5.) 



Limestone and dolomite. (No. 4.) 



Pentamerus-limestone. Corals. (No. 3.) 



Limestone with marly beds. Corals. (No. 2.) 



Brandschiefer. 



Calcareous flagstones or "Pleta." (No. 1.) 



Green sand-beds and chloritic limestone. 



Argillaceous schist. 



Ungulite grit. 



Blue clay and sands. 

 The most ancient beds of the system are seen to subside under 

 No. 1 of Schmidt's Map (and the above list), or summit of the cliffs 

 of the north of Esthonia on the Gulf of Bothnia, known under 

 the name of " Glint ;" and, although occupying no considerable por- 

 tion of the country, those rocks alone have been hitherto chiefly 

 studied. With the exception of the lowest argillaceous and sandy 

 beds, which are well known (as extending from St. Petersburg into 

 the base of these cliffs), the Silurian series of Esthonia is almost 

 entirely composed of calcareous bands more or less pure, which, in the 

 upper strata, often pass into dolomites. The latter varieties, however, 

 occupy no special stratigraphical position, and are not distinguishable 

 through their organic remains from the ordinary limestone with which 

 they are associated. On the contrary, the limestone and the dolomite 

 of the same zone or stage contain precisely the same fossils. 



Each of the five stages indicated in the map prepared by M. Schmidt 

 possesses a peculiar fauna, which is constant throughout the whole 

 of the horizontal extension of the zone. Very few species traverse 

 two entire stages without undergoing what are termed by that author 

 " modifications." Thus, several species belong in common to the two 

 lower bands, — others to the two upper ; but, according to him, one 

 species only, the Calymene Blumenbachii, passes through all the 

 stages, from the first to the fifth inclusive, though in the last of 

 these (the equivalent of the Upper Ludlow rocks) it is modified 

 into the C. spectabilis of Angelin. 



The sharpest separation in the fossiliferous contents of this series 

 occurs between the second and third bands ; thus dividing the 

 "terrain" into Lower and Upper Silurian. These two divisions seem 

 to contain few species common to them, or, to use M. Schmidt's lan- 

 guage, no form which has not been modified. At the junction of these 

 inferior and superior masses there are, however, transitions, which 

 testify that there has been no violent break in the development of 

 animal life, and induce the author to believe, that in this country the 

 Silurian formation is a whole, whose members constitute a compact 

 series of calcareous strata shading into each other, but not conducting 

 by similar points of approximation to the overlying Devonian rocks 

 of Livonia, which occur, he says, as an entirely separate system. 



This sharp distinction is indeed explained by the physical relations 

 of the rocks ; for, whilst the uppermost Ludlow rocks are clearly 

 represented, they are overlaid transgressively by those Devonian 



