Further Exploration of Centre Valley 



the early evening and soon after sunrise. They are 

 well known in most mountain districts as morning and 

 evening stone-showers, and are produced in the following 

 manner. During the day the rocks in these high 

 altitudes (we were now, we estimated, some 8000 feet 

 above sea-level) become strongly heated by the direct 

 rays of the sun, though the clear air, free from dust and 

 containing but little water-vapour or carbonic acid, the 

 great absorbers of radiant heat, remains intensely cold. 

 Thus as soon as the direct rays of the sun are removed 

 from the surface of the rock by the setting of the orb of 

 day, or by the shadow of some mountain peak, it is in 

 the condition of a hot body suddenly plunged in a cold 

 medium. The result is, that its surface layers lose heat 

 rapidly and consequently contract, and as the conduc- 

 tivity of rocks is low, the interior remains hot, with the 

 result that the rapid contraction of the outer chilled layer 

 causes a fracture and the dislodgment of fragments 

 which, falling down the mountain-side and dislodging 

 other loose pieces of rock in their track, form the 

 evening stone-showers. 



The rock-train on the downstream side of the 

 tributary carried by the movement of the ice round the 

 angle separating the valley of the tributary from that of 

 the main glacier, became eventually the lateral rock- 

 train of the latter, while that on the upstream side, after 

 amalgamating with a similar train on the main stream, 

 was carried on down the valley separated from the ice- 

 edge by the ice of the tributary, forming one of the 

 numerous medial rock-trains which we had noticed in 

 such profusion near the termination of the ice above our 

 camp. 



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