378 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 8, 



water might well view these piles of angular blocks, strewed irregu- 

 larly over and lying directly upon the parent rock from which they 

 had been torn up, as having resulted from sudden shocks or jars, 

 which, without throwing up the beds into different and distinct 

 ridges, as in many other parts of the world, simply shattered them 

 into numerous fragments by general convulsive earthquakes which 

 affected broad horizontal areas. But whether or not such a cause 

 be admitted as having had any share in originating the cracks and 

 fissures, the tumultuous condition into which the enormous frag- 

 ments have been thrown may, it seems to me, be best explained by 

 supposing that they were thus piled up in a shallow glacial sea, the 

 ice of which forming in their interstices and around them would, 

 during periodical thaws and debacles, rend the jointed masses 

 asunder, and by its expansion and elevation mingle them in that 

 chaotic fashion which drift ice is so often seen to assume in the 

 polar regions, and which I have myself witnessed on the surface of 

 the great Russian lakes during the spring thaw. In further explana- 

 tion of this view, I simply refer the Society to an explanation which I 

 formerly presented to it of the manner in which rocks in situ on the 

 banks of the Dwina near Archangel are now raised up into ridges 

 of angular materials by the bursting of the river ice, and which, in 

 reference to angular block ridges of former date at different levels 

 along the sides of the lake of Onega, has been illustrated at length in 

 the work upon Russia and the Ural Mountains *. 



Fig, 16. 



River Dwina. 



/. A ledge of carboniferous limestone. a. High bank of sand. 



c. Slope of ditto ditto. e. High water of river. 



b. Broken pile of fragments of ditto. d. Low water of ditto. 



The first of these figures (16) represents the bank of the river 

 Dwina, occupied by a ledge of carboniferous limestone (/), from 

 the upper portion of the sloping talus of which (c) are piles of an- 

 gular fragments (b) which lie between the high water mark(e) and the 

 summer level of the stream {d) ; the angular blocks (6) having been 

 thrown up by the expansion and bursting of the frozen water formed 

 in the interstices of the parent rock. The loose sands (a) have re- 

 sulted from aqueous action of a former period. 



Diagram (17) represents a triple tier of angular blocks (b, 5, b) 

 on the western shore of the lake Onega, south of Petrozavodsk, 

 where the action of frozen water (during northern spring debacles) 

 is presumed to have acted at successive periods, before the lake was 

 reduced from its former levels (c, c, c) to its present level (c?), in 

 the same manner as on the Dwina at the present day. In this ex- 

 ample, the rock is a hard sandstone, not very unlike that of the 



* jLoc. cit. vol. i. p. 568. 



