Septembeb 9, 1910] 



SCIENCE 



323 



pose, then, to ask your attention this even- 

 ing to some aspects of the glacial history 

 of western Europe. 



At no very distant geological epoch the 

 climate in the northern part of the earth 

 was much colder than it is at present. So 

 it was also in the southern; but whether 

 the two were contemporaneous is less cer- 

 tain. Still more doubtful are the extent 

 and the work of the ice which was a conse- 

 quence, and the origin of certain deposits 

 on some northern lowlands, including those 

 of our own islands: namely, whether they 

 are the direct leavings of glaciers or were 

 laid down beneath the sea by floating 

 shore-ice and bergs. Much light will be 

 thrown on this complex problem by en- 

 deavoring to ascertain what snow and ice 

 have done in some region which, during 

 the glacial epoch, was never submerged, 

 and none better can be found for this pur- 

 pose than the European Alps. 



At the present day one school of geol- 

 ogists, which of late years has rapidly in- 

 creased in number, claims for glaciers a 

 very large share in the sculpture of that 

 chain, asserting that they have not only 

 scooped out the marginal lakes, as Sir A. 

 Ramsay maintained fully half a eenturj' 

 ago, but have also quarried lofty cliffs, exca- 

 vated great cirques, and deepened parts of 

 the larger Alpine valleys by something like 

 two thousand feet. The other school, while 

 admitting that a glacier, under special cir- 

 cumstances, may hollow out a tarn or small 

 lake and modify the features of rock 

 scenery, declares that its action is abrasive 

 rather than erosive, and that the sculpture 

 of ridges, crags and valleys was mainly ac- 

 complished in pre-glacial times by running 

 water and the ordinarj^ atmospheric agen- 

 cies. 



In all controvereies, as time goes on, 

 hypotheses are apt to masquerade as facts, 

 so that I shall endeavor this evening to 

 disentangle the two, and call attention to 



those which may be safely used in drawing 

 a conclusion. 



In certain mountain regions, especially 

 those where strong limestones, gran- 

 ites and other massive rocks are dom- 

 inant, the valleys are often trench-like 

 with precipitous sides, having cirques or 

 corries at their heads, and with rather wide 

 and gently sloping floors, which occasion- 

 ally descend in steps, the distance between- 

 these increasing with that from the water- 

 shed. Glaciers have unquestionably oc- 

 cupied many of these valleys, but of late 

 years they have been supposed to have 

 taken a large share in excavating them. 

 In order to appreciate their action we must 

 imagine the glens to be filled up and the 

 district restored to its former condition of 

 a more or less undulating upland. As the 

 mean temperature'' declined snow would 

 begin to accumulate in inequalities on the 

 upper slopes. This, by melting and freez- 

 ing, would soften and corrode the underly- 

 ing material, which would then be removed 

 by rain and wind, gravitation and ava- 

 lanche. In course of time the hollow thus 

 formed would assume more and more the 

 outlines of a corrie or a cirque by eating 

 into the hillside. With an increasing di- 

 ameter it would be occupied, as the tem- 

 perature fell, first by a permanent snow- 

 field, then by the neve of a glacier. 

 Another process now becomes important, 

 that called "sapping." While ordinary 

 glacier-scour tends, as we are told, to pro- 

 duce "sweeping curves and eventually a 

 graded slope," "sapping" produces 

 "benches and cliffs, its action being hori- 

 zontal and backwards," and often domi- 

 nant over scour. The author of this hy- 

 pothesis^ convinced himself of its truth in 



'In the remainder of this address "tempera- 

 ture " is to be understood as mean temperature; 

 The Fahrenheit scale is used. 



'W. D. Johnson, Science, N. S., IX., 1S99, pp^ 

 106, 112. 



