part 1] THROUGH THE ANDES OF PERU AND BOLIVIA. 13 



In comparing the above-described rocks with the granodiorites 

 of the Arica section, the main points of difference to be noticed 

 are the relatively-greater proportion (in the present case) of plagio- 

 clase to orthoclase, the greater abundance of idiomorphic green 

 hornblende, and the frequent occurrence of sphene ; moreover, the 

 acid pegmatite-veins with tourmaline, so characteristically developed 

 farther south, here seem to be absent or at least extremely rare. 



Beyond Cachendo (kil. 55) we reach the Pampa de Isla} 7 , a Hat 

 expanse of desert unbroken in monotony, save for the countless 

 sand-dunes and an occasional outcrop of the underlying rock. This 

 stretches as far as Yitor, where it ends against the foot of the 

 rocky Cerros de la Caldera. 



In the neighbourhood of Huagri occurs a series of isolated 

 rounded hills of reddish hue, formed of granite, which when 

 unweathered, as in the shallow cutting at kilometre 66, is seen to- 

 be a coarsely-crystalline grey rock. It is composed of quartz, 

 felspar (both orthoclase and plagioclase), biotite, and hornblende. 

 This granite is a highly-acid rock, and, as the two felspars are 

 present in roughly equal proportions, it may be described as a 

 typical adamellite. Patches occur, however, composed entirely 

 of quartz and pink orthoclase, with occasional pegmatitic inter- 

 growth, and large plates of biotite often measuring an inch or 

 more in diameter. These patches are occasionally so abundant 

 that the rock presents the appearance of a normal pink granite. 



Smaller basic secretions are also met with. 



(A 73) Adamellite. Huagri (kil. 65). A specimen characteristic of the 

 main bulk of the rock. 

 Microscopic characters : — A holocrystalline plutonic rock, with hypidio- 

 morphic granitic structure ; consisting essentially of quartz, felspar, 

 hornblende, and biotite. 



Clear allotriomorphic quartz is abundant. 



The amount of orthoclase present is equal to, or in slight excess of, 

 the plagioclase ; it occurs in the form of large plates, without definite 

 crystal boundaries, surrounding and enclosing- idiomorphic crystals of the 

 latter (near andesine). This structure is typical of the adamellites, a3 

 distinct from the true granodiorites. 



Green hornblende and brown biotite are present in about equal pro- 

 portions, the two minerals being often intergrown. Minor accessories 

 include magnetite, sphene, and a little apatite. 



After passing La Joya, one notes occasional exposures of" 

 quartzites and sandstones, forming low-lying hills between Ramal 

 and Vitor. These beds doubtless belong to the same series as the 

 quartzites of Posco and Cachendo, but failed to yield any patae- 

 ontological evidence to prove the connexion. 



In the neighbourhood of Vitor, about a mile east of the station, 

 occurs a further isolated outcrop of plutonic rock which has a very 

 distinct individuality ; but, as it is completely surrounded by desert 

 sand, it was found impossible to determine its relation with the rest 

 of the series. Its surface is deeply etched by the natural sand- 

 blast to which it is exposed, and presents a marked difference in 



