Oh. Xn.] GLACIATION OF SCOTLAND. 151 



mark, also, recent shells have been found in stratified beds, closely 

 associated with the boulder clay. 



It was stated that in Russia the erratics diminished generally in 

 size in proportion as they are traced farther from their source. The 

 same observation holds true in regard to the average bulk of the 

 Scandinavian boulders, when we pursue them southward, from the 

 south of Norway and Sweden through Denmark and Westphalia. 

 This phenomenon is in perfect harmony with the theory of ice-islancls 

 floating in a sea of variable depth ; for the heavier erratics require 

 icebergs of a larger size to buoy them up ; and, even when there are 

 no stones frozen in, more than seven-eighths, and often nine-tenths, 

 of a mass of drift-ice is under water. The greater, therefore, the 

 volume of the iceberg, the sooner would it impinge on some shallower 

 part of the sea ; while the smaller and lighter floes, laden with finer 

 mud and gravel, may pass freely over the same banks, and be car- 

 ried to much greater distances. In those places, also, where in the 

 course of centuries blocks have been carried southward by coast-ice, 

 having been often stranded and again set afloat in the direction of a 

 prevailing current, the blocks will diminish in size the farther they 

 travel from their point of departure, for two reasons : first, because 

 they will be repeatedly exposed to wear and tear by the action of the 

 waves ; secondly, because the largest blocks are seldom without divis- 

 ional planes or "joints," which cause them to split when weathered. 

 Hence, as often as they start on a fresh voyage, becoming buoyant by 

 coast-ice which has frozen on to them, one portion of the mass is 

 detached from the rest. An examination which I made in 1852 of 

 several trains of huge erratics in lat. 42° 50' N. in the United States, 

 in Berkshire, on the western confines of Massachusetts, has convinced 

 me that this cause has been very influential both in reducing the size 

 of erratics, and in restoring angularity to blocks which might other- 

 wise be rounded in proportion to their distance from their original 

 starting-point. 



Glaciation of Scotland. — Professor Agassiz, after visiting Scotland 

 in 1 840, came to the opinion that the Grampians had been covered 

 by a vast thickness of ice, and had once, like the Alps, been an inde- 

 pendent centre, whence erratic blocks were dispersed in all directions. 

 Mr. Robert Chambers, in 1848, maintained in like manner that Scot- 

 land had once been " moulded in ice," which had everywhere smoothed 

 and scratched the rocks, and ground them down so as to enlarge and 

 widen many valleys. Mr. T. F. Jamieson, following up the same line 

 of investigation in 1858, adduced a great body of additional facts to 

 prove that the Grampians once sent down glaciers from the central 

 regions in all directions toward the sea. " The glacial grooves," he 

 observed, " radiate outward from the central heights toward all points 

 of the compass, although they do not always strictly conform to the 

 actual shape and contour of the minor valleys and ridges." 



In many part's of Scotland, and conspicuously in the basin of the 



