CHAP. Till. FOSSIL MAMMALIA IN DRIFT OF THE SOMME. 137 



posing them to have been brought by ice, during a general 

 submergence of the country, from some other hydrographical 

 basin. 



But in some of the pits at St. Acheul there are seen in 

 the beds No. 4, fig. 21, not only well-rounded tertiary pebbles, 

 but great blocks of hard sandstone, of the kind called in the 

 south of England "greyweathers," some of which are three 

 or four feet and upwards in diameter. They are usually 

 angular, and when spherical owe their shape generally to 

 an original concretionary structure, and not to trituration 

 in a river's bed. These large fragments of stone abound 

 both in the higher and lower level gravels round Amiens and 

 at the higher level at Abbeville. They have also been 

 traced far uj) the valley above Amiens, wherever patches of 

 the old alluvium occur. They have all been derived from 

 the tertiary strata which once covered the chalk. Their 

 dimensions are such that it is impossible to imagine a 

 river like the present Somme, flowing through a flat 

 country, with a gentle fall towards the sea, to have carried 

 them for miles down its channel, unless ice co-operated 

 as a transporting power. Their angularity also favors the 

 supposition of their having been floated by ice, or rendered 

 so buoyant by it as to have escaped much of the wear and 

 tear which blocks propelled along the bottom of a river- 

 channel would otherwise sufl'er. We must remember that the 

 present mildness of the winters in Picardy and the northwest 

 of Europe generally is exceptional in the northern hemisphere, 

 and that large fragments of gi-anite, sandstone, and limestone 

 are now carried annually by ice down the Canadian rivers in 

 latitudes farther south than Paris.* 



Another sign of ice agency observed by me in many pits at 

 St. Acheul, and of which Mr. Prestwich has given a good 



* Principles of Geology, 9th etL, p. 220. 



