12 MB. J. A. DOUGLAS OX GEOLOGICAL SECTIONS [vol. lxXVl, 



that they are altered Mesozoie sediments, forming a continuation 

 of the Jurassic zone of the south. 



The contact of igneous with sedimentary rock, on which the 

 gold mines of Posco are situated, appears to he an irregular one 

 irrespective of the dip of the strata : a mode of intrusion similar 

 to that described in the case of the batholitic mass of the Llutah 

 and Palca Valleys. 



On our journe}" from the south we met with plutonic rocks of 

 like character in the valley of Moquegua and in the Quebrada de 

 Huaneros, and it is therefore natural to conclude that we are here 

 dealing with the northward prolongation of the same batholite. 

 A definite connexion between the two areas, however, cannot be 

 established, owing to the enormous intervening stretch of desert, 

 the Pampa de Clemesi. 



If this granodiorite of Cachendo were the only plutonic rock in 

 the district, the evidence for such a connexion might reasonably be 

 deemed sufficient. But, between the foot-hills of the coast and the 

 high volcanic peaks of the Western Cordillera, occurs a vast plutonic 

 complex comprising 2-ocks of very varied composition ; and, as many 

 of these crop out, in isolated positions, from beneath the sand of 

 the desert, it is frequently impossible to determine the relationship 

 that they bear one to the other. In the Cerros de la Caldera, 

 however, where the evidence is more easily interpreted, a clearer 

 sequence can be established, in winch at least three distinct 

 phases of plutonic intrusion are represented, separated by definite 

 (though not necessarily long) intervals of time. 



The problem, then, consists in proving which of these intrusions 

 corresponds to, or is continuous with, the batholite of Northern 

 Chile. 



(A 95) Granodiorite. Posco (kil. 40). 



Microscopic characters : — A holocrystalline rock of granitic structure, 

 containing' a certain amount of quartz and a little orthoclase. 



The bulk of the felspar, however, is a plagioclase showing albite and 

 pericline twinning and occasional zonary banding. The extinction-angles 

 indicate an acid labradorite. Green hornblende is abundant in large 

 idiornorphic crystals wbich are frequently twinned. 



The biotite has been almost completely converted into chlorite, with 

 numerous lenticular masses of yellow epidote lying between the cleavage- 

 planes. 



Minor accessories include a little sphene, magnetite, apatite, and a few 

 small crystals of zircon. 



(A 72) Granodiorite. Cachendo (kil. 51). 



This rock is essentially similar to the last, but of somewhat finer grain, and 

 with a more pronounced pink coloration. The phenocrysts of hornblende 

 tend to have a parallel arrangement. 

 Microscopic characters : — Compared with A 95, it is seen to contain more 

 abundant quartz and orthoclase, though the latter is still quite sub- 

 ordinate to the plagioclase, which shows marked zonary banding with 

 frequent alteration of the internal zones. 



The idiornorphic hornblende is twinned parallel to 100. 

 The biotite is fairly fresh, and retains much of its original brown colour, 

 though there is a considerable amount of chlorite and epidote. 



Magnetite is abundant, but apatite and sphene are not well repre- 

 sented. 



