﻿I860.] 



FORBES BOLIVIA AND PERU. 



57 



Orthis Aymara, Orthis (PI. IV. fig. 15), and Cucullella, all in sandy 

 shale, and in a soft blue clay-slate, apparently the equivalent of the 

 uppermost beds of the Millepaya section, the Beyrichia Forbesii, asso- 

 ciated with an abundant small species of Tentaculites, which appeal's 

 to me to be quite different from the Tentaculites swpremus and T. 

 Saienzii of the lower beds, both of which attain a length of occasion- 

 ally more than 2 inches, whereas this small species never appeared 

 to exceed one quarter of an inch. 



Between this last-mentioned place and La Paz, I have not exa- 

 mined the beds for fossils, and only accidentally met with some An- 

 nelid-tubes in blue slate, about thirty miles north of that city : the 

 position of these is uncertain. 



Prom La Paz, to the eastward, as far as a short way below Unduavi, 

 in the Yungas, I was enabled to make the section across the Silu- 

 rian strata between these places, as shown in Section ~No 2. This 

 appears to cut through the whole of this formation, from the upper 

 beds, which before were met with at Millepaya, down to the Lower 

 Silurian slates with Bilobites, a thickness of probably about 15,000 

 feet. 



Starting to the eastward from La Paz, and crossing the great 

 diluvial formation, with its imbedded stratum of trachytic tuff, which 

 in tbe section is seen to be disturbed and dislocated by several faults, 

 we come upon the first appearance of solid rock some miles to the east 

 of Chuquiaguillo, and find it to consist of crumbly and much wea- 

 thered clay-slate, apparently of considerable thickness, and resting 

 upon greyish impure sandstone, which at the river of Taxani is 

 succeeded by an alternating series of shales, slates, and grauwackes 

 or arenaceous beds, the lowest of these being a blue clay-slate of 

 considerable thickness*, in which an anticlinal is seen, bringing the 

 former beds again into sight. I noticed before coming to La Lancha 

 the occurrence of frequent frictional striae and grooves or furrows, 

 the bearings of several of which I found to vary from rT.N.W. to 

 K. and S., and to N.jST.E. At La Lancha these slates again form 

 an anticlinal, which from the precipitous sides of this immense ravine 

 could be accurately delineated ; further up, the clay-slates are very 

 much contorted in the line of bedding ; but towards the summit 

 they become nearly horizontal, or rather slightly basin-shaped ; and 

 in descending, we have the dip always to the eastward until we come 

 very close to Unduavi. To the west of the summit I did not find any 

 fossils ; but shortly after commencing the descent, the sandy shales 

 were full of Annelid-tracks, in such abundance that rarely was a slab 

 found that was not more or less covered with these burrows and 

 markings. At the Mina Emma, a vein of argentiferous galena, 

 running from 20° to 35° E. of N., and with varied but nearly vertical 

 dip, is being worked ; and there I noticed abundant elongated round 

 bodies in the slate, which from their configuration, and from having 

 invariably a hollow tube in the centre, appeared like Orthoceratites : 

 they were composed chiefly of carbonate of iron. 



* On the top of these highly inclined strata is seen a small patch of diluvial 

 conglomerate, apparently a remnant of the formation further to the west. 



