﻿I860.] 



FORBES BOLIVIA AND PERU. 



stone brought up by the porphyries which constitute the greater mass 

 of this hill. This evidence does not appear to me sufficiently con- 

 clusive to warrant its being separated from the other strata, which 

 appear continuous, and which are decidedly of Upper Oolitic age, — 

 more particularly as we have no example of the occurrence of Car- 

 boniferous beds anywhere along the coast of the Pacific in South 

 America. I have therefore classed them along with the Upper 

 Oolitic series until a more careful examination, which I hope soon to 

 make, may afford data for determining their exact position. 



9. Devonian Formation. — The rocks which in Sections Nos. 1 and 

 2 are represented as of Devonian age have only been so coloured since 

 my arrival in England : when these sections were made in Bolivia 

 I had always regarded them as forming part of the Upper Silurian 

 series, and coloured them accordingly. 



I have been induced by Mr. Salter to look upon them as possibly 

 Devonian, although far from being convinced of their being so in 

 reality. The evidence of their geological age is as follows. No fossils 

 were found in the beds of either of these sections by myself-; but 

 M. D'Orbigny cites one single specimen as occurring near Hachecache, 

 a new species of OrtJris, called by him Orihis pectinata, and re- 

 garded as decidedly Devonian by him, although Mr. Salter, judging 

 from the figure and description of M. D'Orbigny, allows that it might 

 pertain to any formation from Silurian to Carboniferous. The Plia- 

 cops Penilandii, from Aygatchi, is from near the junction of these 

 rocks ; but, as previously observed, it may possibly (as Mr. Pentland 

 supposes) come from the base of the Carboniferous basin, the beds 

 at the base of which might consequently be of Devonian age ; but 

 the exact locality of this fossil is too uncertain to allow it to be con- 

 sidered in settling this question. A series of beds of somewhat 

 similar mineral composition occurs in the same strike as these at 

 Oruro, in which a white sandstone contains great numbers of an 

 Orihis considered by Mr. Salter as Devonian or Carboniferous from its 

 belonging to the group of Orthis resupinata and O.filiaria. The Car- 

 boniferous series having been also developed, as previously mentioned, 

 in Oruro, this Orihis might possibly belong to the sandstones at the 

 base of the same. A Favosites also found near Oruro does not afford 

 any satisfactory evidence, and we have only one fossil admitted to be 

 truly Devonian — the Phacops latifrons, found in a rolled pebble in 

 the diluvial plain near Oruro, and which is believed by Mr. Salter 

 to agree in all essential particulars with the European and American 

 species. 



Any evidence derived from thickness of strata in a case where the 

 Devonian, Upper Silurian, and Lower Silurian formations united, as 

 exhibited in the sections here laid before the Society, are not con- 

 sidered to attain a collective thickness of more than 20,000 feet, can- 

 not be taken as in any way conclusive against grouping the whole 

 of these strata under the Silurian formation, when the magnitude 

 of similar strata in other parts of the world is taken into con- 

 sideration. 



Having frequently heard of the immense development and thiek- 



e 2 



