138 Murchison and Verneuil on (he Geological Structure 



erratic blocks, which he can demonstrate were deposited upon the bottom 

 of the sea. 



Angular block-ridges on lake and river Banks. — On the western shore of 

 the great lake of Onega, the attention of the authors was directed by 

 Colonel Armstrong," to three parallel ridges of large angular blocks of 

 hard grit (old red sandstone?), which occur at heights, varying from 20 

 or 30 to 150 feet or more above the level of the water. As these blocks 

 were identical in composition with the solid subjacent rock, and also quite 

 angular, it was at once evident that they had not been drifted, but simply 

 rent from the solid rock which forms that side of the lake. On a first in- 

 spection, the authors were disposed to think that these appearances might 

 have been caused by upheaving or vertical shocks of earthquakes, which 

 they presumed might be among the last signs of the great igneous action 

 which had once been so dominant in these northern tracts ; and they were 

 unable to account for them satisfactorily, until they detected the results 

 of modern action of river ice, which completely explaijied the lacustrine 

 case. 



About 80 miles above Archangel they met with a ridge of large angular 

 blocks of white limestone piled up between the road on which they tra- 

 velled and the river edge, and about 20 or 30 feet above the stream. Hav- 

 ing ascertained that this great river was periodically subject to occasional 

 extraordinary rises in the spring, and that on those occasions it bursts 

 and throws up upon its banks blocks of ice to heights of 20 or 30 feet 

 above its ordinary level, they had at once a solution of the phenomenon ; 

 for the blocks of white limestone had evidently formed parts of the sub- 

 jacent strata, which, projecting into the mud and water on the edge of 

 the Dwina, had been first entangled in ice, and rent ofi" at their natural 

 joints upon the expansion of the ice by which they were upheaved into 

 their present position, taking their present irregular talus shape when the 

 ice melted away from them. Believing, therefore, that the angular ledges 

 on the lake of Onega were similarly formed, the authors see in them the 

 proofs of the lakes of Northern Russia having formerly stood at much 

 higher levels, from which the waters, they suppose, have been let off by 

 successive elevations of the land ; and they further think, that the dimi- 

 nution of shallow lakes, and the conversion of marshes into land within 

 the historic period in Northern Russia, strongly corroborate the rise of 

 this portion of the earth. 



Conclusion. — In recapitulating the chief points of the first and prac- 

 tical part of their Memoir, wherein they establish, they trust on a sound 

 basis, the general classification of the PalcBOzoic rocks of Russia in Eu- 

 rope, the authors remark, that the fact of some of the deposits of such high 

 antiquity being found to stretch in horizontal and almost unbroken sheets 

 over spaces of 1000 miles in length, in a very slightly solidified or lapidi- 

 fied state, is the more interesting when coupled with the absence, through- 

 out the same regions, of all plutonic or igneous rocks. This phenomenon 

 must, it is conceived, exercise considerable influence upon geological 



* Director' of the Impciial Iroa Foundries of Feiraxowodsk. 



