Messrs Peach and Home on the Glaciation of Caithness. 329 



attained its maximum development, all the mountains of 

 Savoy, Dauphiny, etc., had their local and independent 

 glacier systems, some of which were very considerable. 

 These local glaciers flowed down the valleys, as a matter of 

 course. By-and-by, however, when the united glaciers of the 

 Rhone, the Arve, the Isere, and the Drac, with their affluents 

 reached their greatest extension, so as to cover all the region 

 between Bourg, Lyons, Vienna, and Grenoble with a vast 

 mer de glace, the formerly independent glacier systems of 

 Dauphiny, etc., were overwhelmed, and their flow arrested, 

 and in many cases actually reversed. In other words, the 

 united mers de glace of the Ehone, the Arve, etc., sometimes 

 overflowed the summit-levels from which the local glaciers 

 had descended, while in other cases they simply dammed 

 back the local ice and protruded long tongues of ice into the 

 lateral valleys formerly occupied by independent glaciers. 

 And thus alpine rocks were often carried in very different 

 directions to the course followed by the debris of the local 

 moraines. But when the great mer de glace declined in 

 importance, the local glacier systems came again into exist- 

 ence, and rocks of local origin travelled down the valleys as 

 before. 



" German geologists have long been familiar with the fact 

 that ' intercrossings ' of erratics are not uncommon in the so- 

 called Northern Drift ; and I may refer you to ' Prehistoric 

 Europe,' pp. 203, 564, and Plate D, where you will find some 

 account of the general results arrived at. The * intercrossings' 

 of boulders in the drift deposits of Lancashire, Cheshire, etc., 

 so ably described by Mr Macintosh, are, I do not doubt, to 

 be explained in the same way. If those who still cling to 

 the iceberg origin of our boulder drifts can be induced to 

 study MM. Falsan and Chantre's work, they will pause 

 before appealing to the distribution of boulders in the north- 

 west of England in support of the marine theory of the 

 drifts. To me that distribution is eloquent of the successive 

 changes of ice-flow which took place during the gradual 

 increase and decrease of the mer de glace which enveloped 

 that part of England. Long before that mer de glace attained 

 its full development, the glaciers of North Wales and the 



