Vol. 70.] THROUGH THE ANDES OF PERU AXD BOLIVIA. 29 



(A 28 ) Diorite ; Comanche (Bolivia). (PL VII, fig. 5.) 

 Macroscopic characters: A pale-grey holocr ystalline roc k 

 of even-grained medium texture, composed of white plagioclase 

 and dark-green hornblende, the latter frequently segregated 

 into more basic patches. No quartz visible. It often contains 

 abundant micaceous haematite, especially developed along the 

 joint-faces. 



Microscopic characters: Holocrystalline, hypidiomorphic 

 structure. Plagioclase, chiefly andesine and acid labra- 

 dorite, showing Carlsbad and polysynthetic twinning and well- 

 marked zonary structure. The felspar is mostly fresh, but exhibits 

 some alteration to sericite — beginning usually at the centre of 

 the ciystals. Green hornblende occurs in small well-defined 

 c ry stals, with idiomorphic contours . 1 1 m e n i t e altering to s p h e n e, 

 and a little apatite, epidote, and chlorite also are present. 



(2) About 5 miles east of Comanche, the red gypsiferous sand- 

 stones and marls are seen to overlie the rocks of the second series 

 with apparent conformity. The latter, however, I regard as forming 

 a distinct group, the junction being one of transgressive overlap. 

 The original dip of the older beds was probably insignificant in 

 comparison with their present inclination, and the intense Tertiary 

 folding, which affected both Cretaceous and older rocks alike, has 

 enhanced the deceptiveness of this pseudoconformity. 



The rocks of the older series consist of dark-red and chocolate- 

 coloured sandstones (PL VI), with frequent bands of conglomerate ; 

 and, being more resistant than those of the first group, the} r stand 

 out in bolder relief. Thej r are strongly folded throughout, the 

 western limbs of the anticlines having, as a rule, the greater in- 

 clination — in fact, in some instances, they approach a monoclinal 

 structure, the direction of folding having been apparently towards 

 the west. At Coniri, dipping 30° north-eastwards, they end 

 abruptly, along a fault-line, against vertical shales and sandstones 

 of Devonian age. 



The last beds of the series exposed are coarse conglomerates, 

 composed of big well-rounded pebbles, set in a compact reddish or 

 green matrix impregnated with copper. The pebbles consist chiefly 

 of olive-green and liver-coloured quartzites, yellow sandstone, vein- 

 quartz, red crinoidal limestone apparently of Carboniferous age with 

 poorly-preserved fossils (Spirifer sp., Chonetes sp., Euomjjhahtss^., 

 Rhipidomella sp., and a small semireticulate JProductus), white 

 sandstone with a small species of Orthis, pink limestone with 

 Fusulina, and dolomite with Fenes fella. A careful search failed 

 to reveal any trace of granitic or other igneous pebbles, as mentioned 

 by Forbes, although the} r occur abundantly in the alluvial deposits 

 of the valley. That author further describes these conglomerates ' 

 as comprising the lowest beds of the sequence; but it seems 

 clear, from their easterly dip, that they must overlie a considerable 



1 Q. J. G. S. vol. xvii (1861) p. 38. 



