98 Charles Darwin. 



of the geological structure of the entire region was 

 obtained by him is evident. He found shells, that 

 formerly lived in the ocean, in beds fourteen thousand 

 feet above the sea. Here was a bed which he sur- 

 mised was of the age of the chalk cliff of England. 



The altitude affected Darwin not a little, especially 

 in walking rapidly, but he entirely forgot it in the 

 delight of collecting fossils, with which the region 

 abounded. The natives called shortness of breath 

 " puna," and suggested various remedies, having 

 singular and original ideas of the cause. 



The upper peaks here have been called the land 

 of the red snow, and Darwin was constantly on the 

 lookout for its appearance. One day as he was 

 walking behind the mules he noticed in the track of 

 one a reddish line, and then another, until finally 

 the tint was very distinct, and the famous red snow 

 was discovered — a very common phenomenon in 

 this lofty altitude. At first Darwin thought the 

 pale red hue came from the dust, blown from neigh- 

 bouring mountains, of red porphyry. The colour 

 was most distinct when the snow was thawing, and 

 if some of the latter was rubbed on a paper a faint 

 red hue was seen, while under the microscope were 

 found groups of minute spheres in colourless cases, 

 each about a thousandth part of an inch in diameter. 

 The red snow was, in reality, a microscopic plant, 

 Protococcus nivalis, and in the Arctic regions espe- 

 cially it often produces wonderful displays. 



Crossing the mountains the party descended on 

 the other side into a deep valley between the two 

 ranges, and into the republic of Mendoza. The 



