496 Section of the Portillo Chain, paet n. 



surrounding a hummock [Y 2 ], exclusively composed of 

 white granite. Near Los Arenales, the mountains on 

 all sides are composed of the mica-slate ; and looking 

 backwards from this point up to the bare gigantic 

 peaks above, the view was eminently interesting. The 

 colours of the red granite and the black mica-slate are 

 so distinct, that with a bright light these rocks could 

 be readily distinguished even from the Pampas, at a 

 level of at least 9,000 feet below. The red granite, 

 from being divided by parallel joints, has weathered 

 into sharp pinnacles, on some of which, even on some 

 of the loftiest, little caps of mica-schist could be clearly 

 seen : here and there isolated patches of this rock 

 adhered to the mountain-flanks, and these often corre- 

 sponded in height and position on the opposite sides of 

 the immense valleys. Lower down the schist prevailed 

 more and more, with only a few quite small points of 

 granite projecting through. Looking at the entire 

 eastern face of the Portillo range, the red colour far 

 exceeds in area the black ; yet it was scarcely possible 

 to doubt that the granite had once been almost wholly 

 encased by the mica-schist. 



At Los Arenales, low down on the eastern flank, the 

 mica- slate is traversed by several closely adjoining, 

 broad dikes, parallel to each other and to the foliation 

 of the schist. The dikes are formed of three different 

 varieties of rock, of which a pale brown feldspathic 

 porphyry with grains of quartz was much the most 

 abundant. These dikes with their granules of quartz, 

 as well as the mica-schist itself, strikingly resemble the 

 rocks of the Chonos Archipelago. At a height of about 

 1,200 feet above the dikes, and perhaps connected with 

 them, there is a range of cliffs formed of successive 

 lava-streams [A A], between 300 and 400 feet in thick- 

 ness, and in places finely columnar, The lava consists 



