44 ME. J. A. DOUGLAS O^ GEOLOGICAL SECTIONS [April I914, 



Leaving La Paz and ascending the valley of Chuquiagillo, 

 renowned for its rich gold-washings, we soon leave behind the 

 alluvial deposits of the Altaplanicie, and beds of dark slates and 

 quartzites are met with below La Lancha. These at first dip 

 steeply (60°) south-westwards ; but, towards the summit of the 

 pass, they are frequently seen to be much folded. 



With the exception of a few obscure examples of Leptocoelia, no 

 fossils were obtained. Prof. Hauthal, however, has recorded the 

 following from the valley:— Tentaculites sp., Leptocoeliaflabellites, 

 Orthis swbcarinata, Orihotetes chemungensis ; and in addition a 

 more extensive list of Devonian forms from the neighbouring 

 peak of Chacaltaya. These rocks. Avhich were regarded by Forbes 

 as Silurian, have therefore now proved to be of Devonian age. 

 being merely a continuation of those described farther west in 

 the Coniri district. All the species recorded belong to a Lower 

 Devonian facies. 



The upper part of the Chuquiagillo Valley contains much 

 morainic matter and shows abundant proof of recent glaciation. 

 The rocks are smoothed, polished, and striated, jxresenting the 

 typical rounded forms of roches moutonnees. Magnificent 

 examples of rock-steps and barriers are also of frequent occurrence. 



After crossing the pass of Huacuyo, one makes a steep descent 

 clown the U-shaped glaciated valley of Pongo, along the sides of 

 which is exposed a fine section of dark slates and greywackes 

 Avith an occasional band of quartzite. These beds dip almost 

 continuously south-westwards (not eastwards, as stated by Forbes ), 1 

 and therefore underlie the rocks on the western slopes of the 

 range, from which they differ somewhat in general lithological 

 character. With the exception of countless annelid-tracks, no 

 fossils were recorded ; but these slates and greywackes may safely 

 be regarded as constituting an older series than the Devonian, 

 and have therefore been coloured in the section (PI. X, fig. 1) as 

 Silurian. 



Near the custom-house of Unduavi, traces of granite were met 

 with ; to the occurrence of this rock, however, reference will be 

 made later. 



Below Unduavi, the thick vegetation of the tropical forests is 

 encountered, and to prove the geological continuity or relations 

 of such beds as are shown in the rare outcrops at once becomes 

 a matter of extreme difficulty. 



After crossing the spur of Sillutincara, and descending the steep 

 valley of the Chairo River, we observed a thick series of black shales 

 in the river-bed, 1000 feet or more below the Tambo of Huancani. 

 These beds were here found to be almost vertical, with a north- 

 westerly strike. (Although no fossils were found in them, they 

 appear to occupy the same position as the black shales recorded hj 

 Dr. J. W. Evans from the adjacent district of Caupolican, which 

 contain numerous graptolites identified by Mrs. Shakespear as 

 of Arenig age. I have also discovered similar graptolitic shales 

 still farther north, in the Inambari district of Peru.) 



1 Q. .T. G. S. vol. xvii (1861) p. 57. 



