chap. xv. Iquzque, Peru. 575 



throughout the right-hand ridge, the strata dip at 45° 

 to the east : in the left-hand ridge, we have the very 

 same strata and at first with exactly the same dip ; but 

 in following this ridge up the valley, the strata are 

 seen very regularly to become more and more inclined 

 until they stand vertical, they then gradually fall over, 

 (the basset edges forming symmetrical serpentine lines 

 along the crest) till at the very head of the valley they 

 are reversed at an angle of 45° : so that at this point 

 the beds have been turned through an angle of 135°; 

 and here there is a kind of anticlinal axis, with the 

 strata on both sides dipping to opposite points at an 

 angle of 45°, but with those on the left hand upside- 

 down. 



On the Eruptive Sources of the Porphyritic Clay- 

 stone and Greenstone Lavas. — In Central Chile, from 

 the extreme metamorphic action, it is in most parts 

 difficult to distinguish between the streams of porphyritic 

 lava and the porphyritic breccia-conglomerate, but 

 here, at Copiapo, they are generally perfectly distinct, 

 and in the Despoblado, I saw for the first time, two 

 great strata of purple clay-stone porphyry, after having 

 been for a considerable space closely united together, 

 one above the other, become separated by a mass of 

 fragmentary matter, and then both thin out ; — the 

 lower one more rapidly than the upper and greater 

 stream. Considering the number and thickness of the 

 streams of porphyritic lava, and the great thickness of 

 the beds of breccia-conglomerate, there can be little 

 doubt that the sources of eruption must originally have 

 been numerous : nevertheless, it is now most difficult 

 even to conjecture the precise point of any one of the 

 ancient submarine craters. I have repeatedly observed 

 mountains of porphyries, more or less distinctly strati- 

 fied towards their summits or on their flanks, without a 



