﻿24 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



[Nov.. 21, 



coast, and generally having their summits coated with snow. In 

 both Sections (Nos. 1 and 2) this range breaks through the Upper 

 Oolitic series of shales, claystones, porphyry-conglomerates, tuff, and 

 mbedded porphyries, and above Tarata has enclosed, or at all events 

 dislocated, a large mass of these (seen in Section No. 1), in which 

 several strings of copper-ores were noticed : the volcanic rocks here 

 are continuous with those forming the volcano of Chipicani, seen on 

 Section No. 2. This I have not ascended ; I have only passed along 

 the sloping plain at its base, the fragments of volcanic rocks on 

 which left no doubt of its character. Before coming to Uchusuma I 

 noticed a step-like series of trachytic tufaceous beds, so characteristic 

 of this rock when met with in this part of the world, each step being 

 apparently a bed of great extent, and varying in thickness from 10 

 to 30 feet : these are called " Ancomarea " by the Indians, from 

 their white colour ; they extend nearly to the River Cano, at which 

 place their formation is well illustrated in Section No. 2. It would 

 appear that they had been erupted through long narrow fissures or 

 dykes and poured out over the country either as lava or, in some 

 cases, as light volcanic ashes* emitted from the fissures, and de- 

 posited on the ground in their neighbourhood,' where they have gra- 

 dually consolidated into beds. At the Rio Cano two such fissures 

 are seen, bearing nearly E. and W., and dipping 15° to the south, the 

 more western of which can be traced for miles as far as the eye can 

 reach, appearing as a narrow white band or ridge, elevated one or 

 more feet above the ground, from its having resisted the atmosphere 

 better than the porphyry-conglomerate through which it breaks. 

 Sometimes, as at the Rio Mauri (Section No. 2) and Chulluncayani, 

 these are seen capping the rocks, and presenting the appearance of 

 white bands running along the precipitous Hanks of the hills or 

 ravines ; at Pisacorua, Section No. 2, this is also seen, as well as 

 the occurrence of similar bands injected between the beds of the 

 red sandstones, and sometimes continuous for miles. As might be 

 expected, the contrast in colour between these Avhite trachytic rocks 

 and the dark-coloured Oolitic or Bed Sandstone rocks which they cap, 

 or with which they are interstratified, frequently at an immense height 

 up the nearly perpendicular sides of these rugged and barren moun- 

 tains, is wonderfully characteristic, and risible at very great distances. 

 "When breaking through sedimentary rocks, these lateral eruptions 

 appear in general to conform themselves to the fine of stratification, 

 evidently from this affording less resistance, and there being always 

 a much greater tendency for a fissure or crack to follow this Line than 

 to break through the more solid beds. 



Between the Oolitic series and the Permian or Triassic sandstones 

 in Section No. 1, as well as between these last and the Carboniferous 

 basin of the Lake of Titicaca, we find great tracts which to the passer- 

 by present to the eye no signs of other rocks than volcanic, and are 

 occupied by plains or low rounded hills, covered on the surface with 

 abundant fragments of trachyte and trachydolerites or with volcanic 

 alluvium, composed of grains or sand of colourless quartz, white or co- 

 * Frequently containing much puruk-e. 



