138 MOMAY. Chap. XXIII. 



crisp wet ice. I proceeded a mile on it, with much more 

 difficulty than on any Swiss glacier : this was owing to the 

 elevation, and the corrosion of the surface into pits and 

 pools of water ; the crevasses being but few and distant. 

 I saw no dirt-bands on looking down upon it from a point 

 I attained under the red cliff of Forked Donkia, at an 

 elevation of 18,307 feet by barometer, and 18,597 by 

 boiling-point. The weather was very cold, the thermometer 

 fell from 41° to 34°, and it snowed heavily after 3 p.m. 



The strike of all the rocks (gneiss with granite veins) 

 seemed to be north-east, and dip north-west 30°. Such 

 also were the strike and dip on another spur from Donkia, 

 north of this, which I ascended to 19,000 feet, on the 

 26th of September : it abutted on the scarped precipices, 

 3000 feet high, of that mountain. I had been attracted 

 to the spot by its bright orange-red colour, which I found 

 to be caused by peroxide of iron. The highly crystalline 

 nature of the rocks, at these great elevations, is due to the 

 action of veins of fine-grained granite, which sometimes alter 

 the gneiss to such an extent that it appears as if fused 

 into a fine granite, with distinct crystals of quartz and 

 felspar; the most quartzy layers are then roughly crys- 

 tallized into prisms, or their particles are aggregated into 

 spheres composed of concentric layers of radiating crystals, 

 as is often seen in agates. The rearrangement of the mineral 

 constituents by heat goes on here just as in trap, cavities 

 filled with crystals being formed in rocks exposed to great 

 heat and pressure. Where mica abounds, it becomes 

 black and metallic ; and the aluminous matter is crystal- 

 lised in the form of garnets. 



At these great heights the weather was never fine for 

 more than an hour at a time, and a driving sleet followed 

 by thick snow drove me down on both these occasions. 



