324 



SCIENCE 



[N. S. Vol. XXXII. No. 819 



the Sierra Nevada by descending a berg- 

 sehrund 150 feet in depth, which opened 

 out, as is so common, beneath the walls of 

 a cirque. Beginning in the neve, it ulti- 

 mately reached the cliff, so that for the 

 last thirty feet the bold investigator found 

 rock on the one hand and ice on the other. 

 The former was traversed by fracture 

 planes, and was in all stages of displace- 

 ment and dislodgement ; some blocks hav- 

 ing fallen to the bottom, others bridging 

 the narrow chasm, and others frozen into 

 the neve. Clear ice had formed in the 

 fissures of the cliff; it hung down in great 

 stalactites; it had accumulated in stalag- 

 mitic masses on the floor. Beneath the 

 neve the temperature would be uniform, 

 so its action would be protective, except 

 where it set up another kind of erosion, 

 presently to be noticed; but in the chasm, 

 we are informed, there would be, at any 

 rate for a considerable part of the year, 

 a daily alternation of freezing and thawing. 

 Thus the cliff would be rapidly under- 

 mined and be carried back into the moun- 

 tain slope, so that before long the glacier 

 would nestle in a shelter of its own making. 

 Farther down the valley the moving ice 

 would become more effective than sub- 

 glacial streams in deepening its bed; but 

 since the neve-flow is almost imperceptible 

 near the head, another agency must be in- 

 voked, that of ' ' plucking. ' ' The ice grips, 

 like a forceps, any loose or projecting 

 fragment in its rocky bed, wrenches that 

 from its place, and carries it away. The 

 extraction of one tooth weakens the hold 

 of its neighbors, and thus the glen is deep- 

 ened by "plucking," while it is carried 

 back by "sapping." Streams from melt- 

 ing snows on the slopes above the amphi- 

 theater might have been expected to co- 

 operate vigorously in making it, but of 

 them little account seems to be taken, and 

 we are even told that in some cases the 



winds probably prevented snow from rest- 

 ing on the rounded surface between two 

 cirque-heads.^ As these receded only a 

 narrow neck would be left between them, 

 which would be ultimately cut down into 

 a gap or "col." Thus a region of deep 

 valleys with precipitous sides and heads, 

 of sharp ridges and of more or less iso- 

 lated peaks is substituted for a rather 

 monotonous, if lofty, highland. 



The hypothesis is ingenious, but some 

 students of Alpine scenery think more 

 proof desirable before they can accept it 

 as an axiom. For instance, continuous ob- 

 servations are necessary to justify the as- 

 sumption of diurnal variations of tempera- 

 ture sufficient to produce any sensible ef- 

 fect on rock at the bottom of a narrow 

 chasm nearly fifty yards deep and almost 

 enclosed by ice. Here the conditions would 

 more probably resemble those in a glaciere, 

 or natural ice cave. In one of these, dur- 

 ing the summer, curtains and festoons of 

 ice depend from the walls; from them and 

 from the roof water drips slowly, to be 

 frozen into stalagmitic mounds on the 

 floor, which is itself sometimes a thick bed 

 of ice. On this the quantity of fallen rock 

 debris is not greater than is usual in a 

 cave, nor are the walls notably shattered, 

 even though a gap some four yards deep 

 may separate them from the ice. The 

 floors or cirques, from which the neve has 

 vanished, can not as a rule be examined, 

 because they are masked by debris which is 

 brought down by the numerous cascades, 

 little and big, which seam their walls; but 

 glimpses of them may sometimes be ob- 

 tained in the smaller corries (which would 

 be cirques if they could), and these show 

 no signs of either "sapping" or "pluck- 

 ing," but some little abrasion by moving 

 ice. Cirques and corries also not infre- 



* This does not appear to have occurred in the 

 Alps. 



