COINCIDENCE OF DISLOCATIONS AND TRANSVERSE ERUPTIONS. 24* 
In sailing along the Gulf of Finland and passing close under the Isle of Hoch- 
land, we could not avoid there recognising the development of the very same phse- 
nomenon as that which we had witnessed on the Lake Onega. Rising abruptly to 
upwards of 500 feet above the sea, that isle, consisting essentially of porphyritic and 
greenstone rocks, presents a fine rugged outline, the major axis of which ranging 
equally from north-north-east to south-south -west, is parallel to the isles and pro- 
montories of Petrozavodsk, like which it has carried up upon its surface, masses 
oi limestone which have been altered into marble, and sandstones which have been 
changed into quartz rocks 1 . 
The Gulf of Finland, wherein there are three other eruptive islets 2 , is, therefore, 
not merely the line of physical separation between the crystalline rocks of the north 
and the unaltered sedimentary deposits of which we are about to treat, but is also 
distinguished by the same transverse emissions of plutonic matter as the great 
north-eastern boundary. Whether, then, from violent dismemberments and 
changes along the natural frontier of the Azoic and Silurian rocks, which have 
often produced great intervening depressions, or, as in other tracts, from the enor- 
mous accumulation of granitic and crystalline detritus that encumber the surface, 
enough has already been said to explain, why no satisfactory junctions like those 
of Scandinavia can ever be looked for, upon the long frontier line we have been 
considering. 
In the sequel and through many chapters we shall have to treat of very different 
phenomena, and to show, that over the largest region in Europe, which has been 
geologically described by any one set of observers, nearly all the sedimentary masses 
aie unaltered and undisturbed. And here we must request our readers again to 
cast their eyes over the Map and glance southwards from the line of disturbance, 
alteration and fracture to which their attention has been called, and observe, that, 
The structure of Hocliland is described by Professor Hoffman. (See Beitriige zur Kenntniss des 
Russischen Reiches. 4 Bandchen, p. 101. 
M. Baer, who has visited them, thus speaks of these isles in the Gulf of Finland in a letter to Mr. 
Murchison : — “ In the middle of the gulf there are two parallel lines of islets, the one somewhat to the 
north of the other. These latter, embracing the isles of Nerva, Sommer, Hochland and Rodscher, con- 
stitute a series of porphyritic eruptions. Hochland is essentially composed of porphyry and greenstone. 
Sommer and Rodscher contain porphyry only; and though I was unable to land on Nerva, I was suffi- 
ciently near to it to induce me to believe that it also is porphyritic. All these porphyritic isles have worn 
and striated surfaces, here and there covered with a quantity of erratic blocks, among which the variety 
°f Finnish granite called ‘ Rappakivi’ prevails. The southern range of isles nearer to the mainland and 
pax allel to it are mere sandy dunes covered with blocks.” 
E 
