part 1] GEOLOGICAL SECTIONS THROUGH THE ANDES. 29 



In the complete absence of palaeontological evidence, they are 

 here provisionally regarded as forming the continuation of the 

 Mesozoic (probably Jurassic) zone of the foot-hills. 1 



After crossing the divide at Crucero Alto, and continuing in the 

 direction of Lagunillas, we meet with a further series of igneous 

 rocks (at kilometre 204), which appear to be totally distinct from 

 those described above as lying within the volcanic zone of the 

 Western Cordillera. They consist, for the greater part, of dark 

 amygdaloidal basalts, weathering brown or red, and containing 

 abundant traces of copper in the form of carbonates. 



The amygdules are usually filled with calcite or chalcedony, and 

 small porphyritic crystals of augite are also visible in the ground- 

 mass. These lavas are associated with beds of coarse volcanic 

 breccia, which contain blocks of lava, sometimes more than a foot 

 in diameter, set in a matrix of white tuff. Such beds appear to 

 have been formed by a brecciation of the lava in situ. 



Beyond Lagunillas Station a further change takes place in the 

 nature of the landscape : the volcanic covering has entirely disap- 

 peared, and the structure of the underlying country again becomes 

 visible. The railway-cuttings between this point and Saracocha 

 reveal a thick series of grey cherty limestones. These beds cap 

 the hills on the south of the lake, and have, in some cases, the 

 appearance of reef-knolls, occurring in more or less isolated masses 

 which pass laterally into red marls. They are frequently much 

 dolomitized, and included fossils are extremely rare ; a few 

 weathered fragments of echinoderms and lamellibranchs were 

 obtained, however, and these leave little doubt that the limestones 

 are of Cretaceous age, having been formed during the period of 

 wide transgression which is represented in the north by a limestone 

 series of similar appearance, but of extremely fossilifero us character. 



At the base of the Saracocha limestone occurs a thick purple 

 conglomerate, which rests with marked unconformity on an under- 

 lying series of quartzites and shales, with occasional dark-grey 

 limestones (see sketch-map, fig. 2). This conglomerate is well 

 stratified, and frequently contains bands of red marl, devoid 

 of pebbles. Its lowest beds are composed almost entirely of 

 quartzite-pebbles derived from the underlying rocks, Avhile those 

 above contain abundant well-rounded pebbles of igneous origin- 

 (chiefly porphyrites and hornblende-diorites), together with scat- 

 tered subangular and rounded blocks and pebbles of cherty 

 limestone, evidently derived in situ. 



The source of the igneous pebbles is a matter for some speculation. 

 They are composed, for the. most part, of rocks entirely dissimilar 

 to any found in the district, and it is only possible to conclude that 

 they were derived from an older series of plutonic rocks with their 



1 Dr. Erich Jaworski mentions the occurrence of Bajocian-Bathonian beds 

 at Lumbay (Sumbay ?) on the railway from Arequipa to Puno, from the dis- 

 covery of cherty limestone with Nerinea batlionica Rig. & Sauv., Neues. 

 Jahrb. Beilageband xxxvii (1913) p. 305. 



