chap. xv. Iqiiique, Peru. 577 



in the Cordillera, whether anticlinal, uniclinal, or syn- 

 clinal, and as the main valleys have generally been 

 hollowed out along these lines, the intrusive masses 

 have generally suffered much denudation. Hence they 

 are apt to stand in some degree isolated, and to be 

 situated at the points where the valleys abruptly bend, 

 or where the main tributaries enter. On this view of 

 there being a tendency in the old points of eruption to 

 become the points of subsequent injection and dis- 

 turbance, and consequently of denudation, it ceases to 

 be surprising that the streams of lava in the porphyritic 

 claystone conglomerate formation, and in other analo- 

 gous cases, should most rarely be traceable to their 

 actual sources. 



Iquique, Southern Peru. — Differently from what we 

 have seen throughout Chile, the coast here is formed 

 not by the granitic series, but by an escarpment of the 

 porphyritic conglomerate formation, between 2,000 and 

 3,000 feet in height. 1 I had time only for a very short 

 examination ; the chief part of the escarpment appears 

 to be composed of various reddish and purple, some- 

 times laminated, porphyries, resembling those of Chile ; 

 and I saw some of the porphyritic breccia-conglomerate ; 

 the stratification appeared but little inclined. The 

 uppermost part, judging from the rocks near the famous 

 silver mine of Huantajaya, 2 consists of laminated, im- 

 pure, argillaceous, purplish-gray limestone, associated, 

 I believe, with some purple sandstone. In the lime- 

 stone shells are found : the three following species were 

 given me : — 



1 The lowest point, where the road crosses the coast-escarpment, 

 is 1,900 feet by the barometer above the level of the sea. 



2 Mr. Bollaert has described (' Geolog. Proceedings,' vol. ii. p. 

 598) a singular mass of stratified detritus, gravel, and sand, eighty-one 

 yards in thickness, overlying the limestone, and abounding with loose 

 masses of silver ore. The miners believe that they can attribute 

 these masses to their proper veins. 



