GRADUAL MOVEMENT. 199 



into the plains, and at the same time the northern masses of ice 

 advanced southwards. The process must have been very gradual ; 

 and hence the Glacial epoch must have continued for many 

 thousand years. If we assume, in accordance with previous 

 statements (p. 186), that a glacier advances about one league 

 in 50 years, it will have taken one thousand years for the block 

 (Pierre-a-Bot) to be transported from the chain of Mont Blanc 

 through the Trient valley to its present position, and the gra- 

 nite blocks of Seeberg will have passed two thousand years 

 on the glacier in traversing the country from the head of the 

 valley of Ering to the locality where they now are ; the " plough- 

 stone " above Erlenbach will have advanced in the first hun- 

 dred years to the neighbourhood of Glaris, in the second cen- 

 tury it will have approached Urnen, in the third it will have 

 got on as far as Utznach, in the fourth it will have been near 

 Rapperschwyl, and in the sixth century it will have reached its 

 present home. 



Probably the glacier-movement took place much more rapidly, 

 as the speed of advance is in proportion to the size of the glacier ; 

 but these numbers may give an approximate notion of the length 

 of time required for the extension of the glaciers and the trans- 

 port of the masses of rock carried down by them. Additional 

 evidence of the very slow movement of the glaciers in their in- 

 vasion of the highlands is given by the stratified drift. This 

 chiefly consists of Alpine rocks, and must therefore have been 

 dispersed during the Glacial epoch. The immense pebble-beds 

 which overlie the lignites of Utznach were very probably depo- 

 sited at a time when the glacier occupied the valley between 

 Utznach and the Buchberg, bringing with it, in its lateral 

 moraines, a mass of material which was swept away by the 

 streams flowing from the glacier, and was spread over the area 

 of the lignites. Let it also be remembered that the water flow- 

 ing from the right mountain-side must have been dammed up 

 by the glacier that filled the valley, and no doubt here and 

 there formed small pools, whilst in other parts the stream ran 

 along the side of the glacier, and thus probably assisted in 

 the dispersion and deposition of the rubbish of the glacier. 

 When the glacier afterwards rose, it covered this stratified 



