﻿I860.] 



FORBES BOLIVIA AND PERU. 



35 



ceeded by a second series of thick porphyry- conglomerates of the 

 same character as the former ones. In the beds of porphyry-tuff 

 I noticed fissures filled with a crystalline zeolitic mineral, probably 

 stilbite *. 



These porphyry-conglomerate beds continue up to the valley of 

 Pailumani, where they are cut through by the great volcanic mass of 

 felspathic lava seen in the section ; and no trace was then found of 

 them before coming to the eastern slope of the Pass of Chulluncayani, 

 where we again meet with a series of porphyries which appear to 

 belong to this series, and on the top of which I found several 

 patches of altered red sandstone near Condorana, which evidently 

 belonged to the Permian or Triassic series further to the east, and 

 appear to have been carried up by the eruption of these porphyries. 



In Section No. 1 another transverse view of the stratification of 

 this series is obtained, which, however, is not so extensive as the one 

 just described, owing to the protrusion of the great mass of volcanic 

 matter to the eastward. The rocks met with in this section are 

 precisely similar in mineral character to those met with and de- 

 scribed in the former Section (No. 2), being composed of argillaceous 

 shales, porphyry-tuffs, conglomerates, claystones, mudstones, and 

 interstratified porphyries, cut through by dioritic and volcanic rocks, 

 and at the western extremity of the section dislocated by a series of 

 faults, which are easily observed on the nearly perpendicular sides of 

 the great ravine which forms a passage through this chain from 

 Quilla to the plains of Sama. They are seen to great advantage, and 

 were easily sketched and followed out, from the occurrence of several 

 bands or beds of different colours and consistency, amongst which 

 several thick beds of coarse porphyry-conglomerate were very cha- 

 racteristic. This section itself will, it is believed, not require further 

 description, as the general relations of the strata are not very com- 

 plicated. 



At the Morro de Arica, a hill situated to the immediate south of 

 the town of Arica, and rising perpendicularly from the sea to a 

 height of about 500 feet above the water's level, we also find a series 

 of porphyries interstratified with sedimentary beds, but the age of 

 which has not been as yet satisfactorily determined. 



These beds are coloured by M. D'Orbigny as Carboniferous, from 

 his having found fragments of Procluctus in limestone boulders 

 enclosed in the porphyry of this hill. I have not considered it ad- 

 visable at present, before more data are obtained, to separate them 

 from the other strata with which they appear continuous, and which 

 have yielded Liassic remains ; but I admit that it requires more careful 

 examination. A sketch-section of this hill, taken by me on my first 

 visit to Arica in 1857, shows the following features. 



Commencing from below upwards, we find at the base a series of 

 much-burnt and altered shales, thin-bedded, and of a brown colour, 

 but too much altered to admit of any recognition. Above this is a 



* On examination, its specific gravity was found to be 2-14, the percentage of 

 water contained in it 17'62, and its hardness 3 - 2o ; before the blowpipe it intu- 

 mesces, becomes milk-white, and ultimately fuses into a white enamel. 



D 2 



