chap. xv. near Castano. 569 



and that the one fragment of mica- slate, and the pebbles 

 of quartz low down in the gypseous formation at Las 

 Amolanas, have been derived from it. The mica-schist 

 is succeeded by stratified porphyritic conglomerate [Q] 

 of great thickness, dipping eastward with a high in- 

 clination : I have included this latter mountain-mass 

 in the same anticlinal axis with the porphyritic streams 

 [N N] ; but I am far from sure that the two masses 

 may not have been independently upheaved. 



Seventh Axis of Elevation. — Proceeding up the 

 ravine, we come to another mass [R] of andesite ; and 

 beyond this, we again have a very thick, stratified por- 

 phyritic formation [S], dipping at a small angle east- 

 ward, and forming the basal part of the main Cordillera. 

 I did not ascend the ravine any higher; but here, 

 near Castano, I examined several sections, of which I 

 will not give the details, only observing, that the por- 

 phyritic beds, or submarine lavas, preponderate greatly 

 in bulk over the alternating sedimentary layers, which 

 have been but little metamorphosed : these latter con- 

 sist of fine-grained red tuffs and of whitish volcanic 

 grit-stones, together with much of a singular, compact 

 rock, having an almost crystalline basis, finely brec- 

 ciated with red and green fragments, and occasionally 

 including a few large pebbles. The porphyritic lavas 

 are highly amygdaloidal, both on their upper and 

 lower surfaces; they consist chiefly of clay-stone por- 

 phyry, but with one common variety, like some of the 

 streams at the Puente del Inca, having a gray mottled 

 basis, abounding with crystals of red hydrous oxide of 

 iron, green ones apparently of epidote, and a few glassy 

 ones of feldspar. This pile of strata differs consider- 

 ably from the basal strata of the Cordillera in Central 

 Chile, and may possibly belong to the upper and 

 gypseous series: I saw, however, in the bed of the 



