﻿I860.] 



FORBES — BOLIVIA AND PERU. 



37 



entire thickness in Sections Nos. 1 and 2, in which, although at first 

 sight thej seem of much greater magnitude, they do not in reality 

 appear anywhere to attain a maximum thickness of more than 

 6000 feet, and generally are found to be much under this estimate. 

 This is due to the great number of folds doubling up the beds, 

 and also, as seen in Section No. 2 (from Santiago to Nasacara), to 

 a series of faults which repeatedly bring up the same beds to the 

 surface. 



D'Orbigny has in his section across the same line of country 

 coloured the greater part of these beds as of Devonian age : at Coro- 

 coro he makes a part of them Carboniferous, and at the Disaguadero 

 (Nasacara) puts in a little strip of Triassic. As he cites no fossil 

 evidence for these divisions, and in fact admits that he has no fossils 

 whatever from any part of this section, this cutting up into forma- 

 tions beds conformable to one another, and strikingly analogous in 

 mineral composition, seems unexplainable except by imagining that 

 here, as generally throughout his ' Geology of Bolivia,' he proceeds 

 with the supposition that no link in the chain of geological forma- 

 tions should be deficient. 



The strata so classed under these different denominations can in 

 some instances be shown to be part of one and the same series of 

 beds, and, taken as a whole, possess all main features in common. 



I have therefore not considered myself justified in retaining these 

 subdivisions, until at least more evidence is produced, and for the 

 present have grouped the whole of these beds as one series, under 

 the name of Permian or Triassic. The balance of evidence appears 

 in favour of the Permian epoch, although at the same time I admit 

 that the absence of satisfactory fossil-evidence still leaves the ques- 

 tion an open one for inquiry. 



These beds are penetrated, upheaved, and altered by the linear 

 eruption of dioritic rocks which runs through the whole extent of 

 Peru, Bolivia, and Chile, and which are contemporaneous with the 

 Cretaceous period. The section from Pisaca to Comanche shows 

 an example of this. That they are more ancient than the Upper 

 Oolitic series, is shown by their having been broken up and ele- 

 vated by the porphyries which are found imbedded or inter- 

 stratified in this system, and are inseparably connected with the 

 same in geological age. At Condorana (Section No. 2) this will be 

 seen to be the case. Still further north, between Condorana and 

 Pisacoma, these beds appear to dip beneath the whole Oolitic series ; 

 but the nature of the ground was not favourable to a perfectly con- 

 clusive examination. 



Fossil plants are everywhere found in this formation ; but gene- 

 rally they are very indistinct. In some places, as at Pontezuelo, large 

 trunks of trees silicified are found in abundance ; and several speci- 

 mens of carbonized wood which I procured from the sandstones of 

 Corocoro are as yet not examined *. 



* Since the above was written, sections have been made of two of these woods, 

 and prove them to be Coniferous ; but the structure is too indistinct to allow of 

 further recognition. 



