Ice and its Natural History 275 



visited the island of San Antonio in the Cape Verde Group. 

 Here rain is very scarce. It was said that it had not rained 

 for three years. He landed opposite the entrance of the valley 

 of Tarrafal and took a photograph, in which the dry river-bed, 

 encumbered with rocks and stones, occupied the foreground. 

 About noon a cloud-burst occurred from which it was ne- 

 cessary to take shelter for a considerable time. On returning 

 to the landing-place to rejoin his ship, he found that the 

 stream had cut out a broad bed in the stones more than 

 a metre in depth and precipitated the whole of the debris 

 into the sea. He photographed the river-bed after this 

 catastrophe, and the pair of photographs constitutes one of 

 the most important documents in the natural history of de- 

 nudation. By the kindness of His Highness, I am permitted 

 to reproduce these photographs. Fig. 6a represents the view 

 in the morning. Dr Portier, the well-known bacteriologist, is 

 standing on the gravel plain in front of the larger boulder, 

 and the whole of his body is visible over the smaller boulder 

 in the immediate foreground, which is nearly covered by the 

 gravel. Fig. 6b represents the same view in the evening, after 

 the stream had done its work of distributing the accumulation 

 of debris due to three years' chemical and gravitational de- 

 nudation. All that is now seen of Dr Portier, over the top 

 of the nearer boulder which has been almost completely 

 undermined by the torrent, is the top of his helmet. The 

 stones in the river-bed in the morning were as round as those 

 occurring in any perennial river, yet it was impossible in this 

 case for the river to have had anything to do with the rounding 

 of them. The sole function of the river in this island is, once 

 every year or so, to clear the bed of rounded stones, not to 

 make them. 



It must be observed that, in the island of San Antonio, 

 though rain is rare moisture is abundant, and the valleys are 

 fertile and well cultivated. The moisture condensed on the 

 ground, and retained beneath the surface, aided by the high 

 temperature due to the low latitude, produces ideal conditions 

 for the chemical decomposition of the volcanic rocks of which 

 the island consists, and the consequent production of the best 



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