Dr, A. P. Young—Glaciation of Navis Valley. 207 
who have noted the highest occurrences of ice scratches and erratics in 
different parts of a valley. In the application of this method some 
caution seems to be necessary. On the analogy of present conditions 
and on the ground of direct observations, it may be admitted that the 
nival gradient was, at least in the higher ground, always as steep as 
it is to-day. It is perhaps not out of the question that massive ice 
formed above the snow-line even in the high: ground. But if the rise 
in the snow-line was due to diminished precipitation it is difficult to 
suppose that the firn-line stood very much higher than the snow-limit. 
Eyen assuming for the maximum of cold a somewhat steep nival 
eradient and a firn-line high above the snow-line, it can hardly be 
doubted that ice would form at successively higher levels during the 
retreat. The highest scratches may thus belong to milder periods; 
so too the highest boulders in ground near the centre of glaciation. 
The highest crystalline erratics in the neighbourhood of the Alpine 
foot-hills no doubt belong to a period of extreme cold, but not 
necessarily to the last maximum ; boulders of earlier glaciations might 
persist for an indefinite time if landed between the tributary water- 
courses on shoulders and spurs where they would not be disturbed by 
the ice of a subsequent advance. 
The inclusion in the estimate of boulders and scratches belonging to 
retreating stages would give for the maximum an ice-gradient steeper 
than was actually attained in the higher parts of the valleys. To 
include in the estimate boulders which were landed during some 
older glaciation when the valley-floor was higher would lead to false 
conclusions as to the thickness of the ice in the lower reaches of the 
valleys. 
RELATION BETWEEN Nivat Prane anp Zone or Maximum 
PRECIPITATION. 
The hypothesis that the rise in the snow-limit is mainly the result 
of diminished precipitation in the high ground leads to some interesting 
deductions concerning the relation between nival surface (N.S.) and 
belt of maximum rainfall (M.P.). At the present time the nival 
Fie. 3. Diagram showing break in the nival gradient when the nival surface passes 
through the zone of maximum precipitation. 
surface (N.S. 1, Fig. 3) on the northern slopes of the Alps is nearly 
a plane,’ all parts of which are above the zone of maximum rainfall.’ 
With these conditions as a starting-point we may consider the following 
hypothetical cases :— 
Case 1. If while M.P. remains stationary N.S. is depressed 
continuously, it takes up successive positions as N.S. 2, nearly 
1 Penck & Brtickner, Die Alpen im Hiszeitalter, p. 604. 
? See Hess’s map, loc. cit., p. 74. 
3 Hess, loc. cit., p. 44; J. Hann, Kimatologie, 1905, p. 262. 
DECADE V.—VOL. VII.—NO. VI. 17 
