V GLACIAL EROSION OF LAKE BASINS 123 



sufficient incline to flow onward as it did, it was probably 

 5,000 or 6,000 feet below Martigny and 4,000 or 5,000 

 feet over the middle of the lake. It is certain, at all 

 events, that whatever thickness w^as necessary to cause 

 onward motion that thickness could not fail to be produced, 

 since it is only by the onward motion to some outlet or low- 

 land where the ice can be melted away as fast as it is 

 renewed that indefinite enlargement of a glacier is avoided. 

 The essential condition for the formation of a glacier at all 

 is, that more ice should be produced annually than is 

 melted away. So long as the quantity produced is on the 

 average more than that melted, the glaciers will increase ; 

 and as the more extended surface of ice, up to a certain 

 point, by forming a refrigerator helps i ts own extension, a 

 very small permanent annual surplus may lead to an enor- 

 mous extension of the ice. Hence, if at any stage in its 

 development the end of a glacier remains stationary, 

 either owing to some obstacle in its path or to its having 

 reached a level plain where it is unable to move onward, 

 the annual surplus of ice produced will go to increase the 

 thickness of the glacier and its upper slope till motion is 

 produced. The ice then flows onward till it reaches a 

 district warm enough to bring about an equilibrium 

 between growth and dissolution. If, therefore, at any 

 stage in the growth of a glacier a thickness of six, seven, 

 or even eight thousand feet is needed to bring about this 

 result, that thickness will inevitably be i:)roduccd. We 

 know that the glacier of the Rhone did move onward to 

 the Jura and beyond it; that the northward branch 

 flowed on beyond Soleure till it joined the glacier of the 

 Rhine ; and that its southern branch carried Alpine 

 erratics to the country between Bourg and Lyons, more 

 than 200 miles from its source. We know, too, that 

 throughout this distance it moved at the bottom as well 

 as at the top, by the rounded and polished rocks and beds 

 of stiff boulder clay which are found in almost every part 

 of its course. 



In view, therefore, of the admitted foots, all the ob- 

 jections alleged by the best authorities are entirely 

 wanting in real force or validity ; while the enormous size 



