18091842] VOYAGE 21 



and a confusion of distances and a sort of stillness which gives Letter 6 

 the sensation of being in another world, and when to this is 

 joined the picture so plainly drawn of the great epochs of 

 violence, it causes in the mind a most strange assemblage 

 of ideas. 



The formation I call Porphyritic Conglomerates is the 

 most important and most developed one in Chili : from a 

 great number of sections I find it a true coarse conglomerate 

 or breccia, which by every step in a slow gradation passes 

 into a fine claystone-porphyry ; the pebbles and cement 

 becoming porphyritic till at last all is blended in one compact 

 rock. The porphyries are excessively abundant in this chain. 

 I feel sure at least f ths of them have been thus produced from 

 sedimentary beds in situ. There are porphyries which have 

 been injected from below amongst strata, and others ejected, 

 which have flowed in streams ; it is remarkable, and I could 

 show specimens of this rock produced in these three methods, 

 which cannot be distinguished. It is a great mistake con- 

 sidering the Cordilleras here as composed of rocks which 

 have flowed in streams. In this range I nowhere saw a 

 fragment, which I believe to have thus originated, although the 

 road passes at no great distance from the active volcanoes 

 The porphyries, conglomerate, sandstone and quartzose sand- 

 stone and limestones alternate and pass into each other many 

 times, overlying (where not broken through by the granite) 

 clay-slate. In the upper parts, the sandstone begins to 

 alternate with gypsum, till at last we have this substance of a 

 stupendous thickness. I really think the formation is in some 

 places (it varies much) nearly 2,000 ft. thick, it occurs often 

 with a green (epidote ?) siliceous sandstone and snow-white 

 marble ; it resembles that found in the Alps in containing 

 large concretions of a crystalline marble of a blackish grey 

 colour. The upper beds which form some of the higher 

 pinnacles consist of layers of snow-white gypsum and red 

 compact sandstone, from the thickness of paper to a few feet, 

 alternating in an endless round. The rock has a most curiously 

 painted appearance. At the pass of the Peuquenes in this 

 formation, where however a black rock like clay-slate, without 

 many laminae, occurring with a pale limestone, has replaced 

 the red sandstone, I found abundant impressions of shells. 

 The elevation must be between 12 and 13,000 ft. A shell 



