Vol. 70.] THROUGH THE AIS'DES OF PERU A2TD BOLIVIA. 13 



Augite colourless to pale green, occurring as small irregular 

 grains in the ground-mass with no definite crystal outlines. 



Ilmenite in black grains, altering into spliene. Chlorite- 

 abundant, in irregular yellowish-green pleochroic patches, which 

 wrap round the felspar-laths so as to suggest an original ophitic 

 structure. Epidote and calcite occur sparingly. 



As we pass inland from the coastal section, the whole surface of 

 the country traversed by the railway is covered by sandy deserts 

 formed by the weathering of a thick mantle of volcanic lava and 

 tuff, and all traces of the underlying Mesozoic sediments are com- 

 pletely obscured, except along the valleys of the Llutah and Palca 

 Rivers, which have been eroded to a sufficient depth to bring the 

 Mesozoic deposits once more to light. 



The Llutah enters the sea a few miles north of Arica, and, on 

 proceeding up its valley, we first meet with stratified Mesozoic rocks 

 between Molino and Cata : the}' consist of a thick series of barren 

 black, brown, and olive-green shales having a more or less constant 

 westward dip, and much altered by the intrusive core of plutonic 

 rock, with the production of numerous mineral veins. A thick 

 vertical succession was passed over in ascending the cuesta of 

 Cuescolla to Socoroma, but no evidence of fossil remains was 

 discovered. 



Farther east, at Ancolocalla, similar altered shales and thin 

 limestones, dipping 50° south-westwards, are again visible at the 

 very bottom of the Jamiraya gorge, which is some 5000 feet deep 

 at this point. Here faint traces of fossils were recorded — chiefly 

 small Posidonomyce, similar to those obtained from the Morro 

 de Arica. Penetrating the shales at this locality and cutting 

 across their bedding-planes was found a thick dyke or intrusion 

 of quartz - hypersthene - norite, the true dimensions of 

 which, owing to the inaccessibility of the gorge, could not be 

 determined. The occurrence of this rock is of considerable interest, 

 for it may possibly represent the original source from which the 

 pillow-lavas of the coast-section were extruded, and thus be 

 regarded as their plutonic equivalent. It is absolutely distinct 

 from the intrusive core of granodiorite described on p. 14, which 

 appears to be of much later date ; and I know of no other 

 similar occurrence in any part of the country. 



(A ir ) Quartz - hypersthene - norite ; Ancolocalla, Llutah 

 River (Chile). 



Macroscopic characters: A holocrystalline, compact, even- 

 grained, grey to black rock, composed of plagioclase, slightly 

 iridescent, and black pyroxene, with a little quartz. It is 

 difficult to discriminate between the constituent minerals. (Specific 

 gravity = 2-77.) 



Microscopic characters: Holocrystalline, of somewhat coarse 

 texture, composed essentially of plagioclase, rhombic and mono- 

 clinic pyroxenes, quartz, and iron-ores. 



