1916.] THE DEPOSITS. 369 



Guernsey. The Herrn rocks contain nodules of harder 

 materials from which the looser crystals have started in the 

 solidification of the rock masses, and these nodules refused to 

 decompose, and during the movement of the ice they lost all 

 the softer skins and rounded into true boulders. These 

 boulders were, then, the result of conchoidal weathering then 

 of rolling by ice into round or oval smoothed stones having 

 the appearance of huge pebbles. 



It is worth a visit to Herm to see these boulders, which 

 must have rested thickly on the top of the island, as they are 

 now arranged by some proprietor, into a dry wall. 



There are rocks of the same kind, which weather con- 

 choidally in Guernsey, but these are at too low a level to have 

 been rounded by ice, and they offer the contrast of being rough 

 and irregular in the shedding of their skins. 



The ice-cap of Guernsey produced boulders in large 

 numbers, but they were rolled out of the torn masses of rock 

 and are not as globular as the Herm ones. 



I have no evidence to offer as to the elevation of the 

 island at this stage, but there is indisputable evidence that the 

 ice-cap fed the valleys and eroded them. 1 think, therefore, 

 that a moderate elevation seems all that is indicated. What- 

 ever the height the island was raised to matters little from the 

 insular point of view, for if it was raised say, 30 to 50 feet, 

 above its present level the rise would have been sufficient, 

 under the glacial conditions then existing, to have frozen the 

 lower rocks and forcibly detached the blocks of rock described 

 in the last part. 



The elevation completed, the reverse motion set in and a 

 progressive sinking began. 



The British Islands were by now covered by an enormous 

 mass of ice (see Geikie " Great Ice Age "), and it is thought 

 that the weight of the ice was the cause of the uplift 

 which occurred here. This was effected in two ways (a) the 

 weight of the ice depressing the land in the extreme JNorth, 

 and (1)) the lowering of the sea level by the removal of water 

 from the sea and locking it up on the land. 



The continuation of this cause soon had, however, a 

 contrary effect, for as the ice increased in bulk and weight, it 

 also covered the land to much lower latitudes, and the southern 

 portion of these lands surfaces, feeling the weight, also sank. 



Guernsey was involved in this later movement and also 

 began to sink. At first, only the outside of the united 

 islands (Guernsey, Herm and Jethou) came in contact with 

 the floating ice-floes which formed on the margins of the 



