chap. xv. Gypseous Formation, 557 



and black calcareous shaly rocks, alternating together. 

 The white indurated tuff, bed (11), here contains little 

 or no gypsum, whereas on the northern and opposite 

 side of the valley, it is of much greater thickness and 

 abounds with layers of gypsum, some of them alterna- 

 ting with thin seams of crystalline carbonate of lime. 

 The uppermost, dark coloured, hard mudstone (bed 12) 

 is in this neighbourhood the most constant stratum. 

 The whole series differs to a considerable extent, 

 especially in its upper part, from that met with at 

 [B B], in the lower part of the valley : nevertheless, I 

 do not doubt that they are equivalents. 



Fourth Axis of Elevation (Valley of Coipia'po). — 

 This axis is formed of a chain of mountains [F], of 

 which the central masses (near La Punta) consist of 

 andesite containing green hornblende and coppery mica, 

 and the outer masses of greenish and black porphyries, 

 together with some fine lilac-coloured clay-stone por- 

 phyry ; all these porphyries being injected and broken 

 up by small hummocks of andesite. The central great 

 mass of this latter rock, is covered on the eastern side 

 by a black, fine-grained, highly micaceous slate, which, 

 together with the succeeding mountains of porphyry, 

 are traversed by numerous white dikes, branching from 

 the andesite, and some of them extending in straight 

 lines, to a distance of at least two miles. The mountains 

 of porphyry eastward of the micaceous schist soon, but 

 gradually, assume (as observed in so many other cases) 

 a stratified structure, and can then be recognised as a 

 part of the porphyritic conglomerate formation. These 

 strata [G] are inclined at a high angle to the SE., and 

 form a mass from 1,500 to 2,000 feet in thickness. 

 The gypseous masses to the west already described, dip 

 directly towards this axis, with the strata only in a few 

 places (one of which is represented in the section) 



