A GEOGRAPHICAL SKETCH OF THE JUAN FERNANDEZ ISLANDS 



f47 



Fig. 62. Slope of Mt. Inocentes seen from Cordon Atravesado, c. 1350 m ctouvc sea level. — 



Photo C, Skottsberg ^^/j 191 7. 



resemble the hard basalts between 400 and 600 m on Masatierra (I.e.). The sum- 

 mit of Mt. Inocentes consists of a still different rock, a slaggy olivine basalt 

 supersaturated with iron oxides (I.e. p. 70). The extension and thickness of this 

 bed is unknown. 



As on Masatierra numerous vertical dikes traverse Masafuera, in this case 

 in W — E direction. The rock is extraordinarily rich in olivine and the dikes 

 may represent, at least in part, channels for the beds forming the summit of 

 the island (I.e. p. 74). 



The difference in geological structure between the two islands helps, I think, 

 to explain the profound difference in morphology. The streams, eroding deeper 

 and deeper into the land, have excavated valleys which, in their distal parts, 

 are V-shaped, with steep sides, which rapidly become steeper inland, where the 

 perfect U-shape is retained in the canyon, with the entire narrow width of the 

 gently sloping bottom occupied by the streambed. In the innermost part, where 

 the land reaches its greatest elevation and the precipitation its highest figures, 

 lateral erosion has widened the valley and a fan-shaped series of hanging gorges 

 has been formed. The canyon ends in a high, almost vertical wall with a water- 

 fall. Figs. 63 — 65, 68 — 71, 73, 75 and 'j'j serve to illustrate the valley formation. 

 In the northern, drier and less high half of the island no deep canyons were 

 formed. The ridges left standing between the valleys are very unlike the ridges 



