part 1] THROUGH THE ANDES OE PERU AND BOLIVIA. 33 



large intrusive masses of diorite. The diorite of Maravillas, both 

 in petrographical characters and in mode of occurrence, is essentially 

 similar to the rock previously described by me from Comanche in 

 the Arica-La Paz section ; moreover, though the country-rock is of 

 an entirely different nature in the two areas (in the former case being 

 a red sandstone, the ' Puca ' sandstone of Steinmann), the intrusions 

 are associated in both instances with productive copper-ores. We 

 may reasonably conclude, then, that we are here dealing with the 

 northward prolongation of that post-Cretaceous line of dioritic 

 intrusion, which runs through Comanche and the Cerro de las 

 Esmeraldas farther south in Bolivia ; a line which is probably 

 continued still farther northwards into the district drained by the 

 Apurimac Piver. 



It is interesting to note that the rocks in the immediate neigh- 

 bourhood of the diorite do not appear to be cupriferous, and the 

 mines are usually situated at some little distance from the actual 

 contact. This fact has led some observers to regard the occurrence 

 of the ore as being in no way connected with the igneous intrusion. 

 After visiting both districts, however, I find it impossible to uphold 

 such a theory. 



The association of these rich copper deposits with the diorite is 

 too striking to admit of any other explanation than that they 

 were produced as a direct result of the intrusion ; and, in view of 

 the known distribution of the ore at some distance from the outcrop 

 of plutonic rock, the absence of the latter from the mines of Coro 

 Coro is an argument in favour of, rather than against, this theory : 

 for, if we judge from the outcrops at Comanche and the Cerro de 

 las Esmeraldas, we may reasonably infer that the diorite here lies 

 at no great depth below the surface. 



Although many of the mines have been worked ever since the 

 time of the Spanish conquest, and in some cases previous to that 

 date, the recent discovery of another rich ore-bearing district during 

 the construction of the Arica-La Paz railway shows that the 

 intervening country has not yet been thoroughly prospected with 

 a proper appreciation of the facts. 



(A 118) Diorite. Maravillas. 



A grey holocrystalline rock of medium texture, almost identical in ap- 

 pearance with the Comanche diorite of Bolivia. 1 It consists essentially 

 of white plagioclase and dark-green hornblende with, in the present case, 

 a few flakes of biotite in addition to the latter. Small rounded patches 

 of more basic composition, having the appearance of cognate xenoliths, 

 are of common occurrence. 



Microscopic characters : — A thin section of the rock, when viewed be- 

 tween crossed nicols, has a very peculiar appearance. This is due to the 

 presence in great abundance of minute rounded and irregular grains of 

 quartz, which seem to have been derived from the sedimentary rocks 

 (chiefly Devonian quartzites), through which the dioritic magma has made 

 its way towards the surface. 



The plagioclase (an acid labradorite) shows little signs of alteration, 



1 See Q. J. G. S. vol. lxx (1914) p. 29 & pi. v. 

 Q. J. G. S. No. 301. d 



