Ch. XXXIV.] PLUTONIC ROCKS OF THE ANDES. 577 



referred to the Eocene period, and it follows that those vast movements 

 which have raised fossiliferous rocks from the level of the sea to the 

 height of more than 10,000 feet above its level have taken place since 

 the commencement of the tertiary epoch. Here, therefore, if anywhere, 

 we might expect to find hypogene formations of Eocene date breaking 

 out in the central axis or most disturbed region of the loftiest chain in 

 Europe. Accordingly, in the Swiss Alps, even the fit/scJi, or upper por- 

 tion of the nummulitic series, has been occasionally invaded by plutonio 

 rocks, and converted into crystalline schists of the hypogene class. 

 There can be little doubt that even the talcost- o;rauite or gneiss of Mont 

 Blanc itself has been in a fused or pasty state since the Jlysch was de- 

 posited at the bottom of the sea ; and the question as to its age is not so 

 much whether it be a secondary or tertiary granite, or gneiss, as whether 

 it should be assigned to the Eocene or Miocene epooh. 



Great upheaving movements have been experienced in the region of 

 the Andes, during the Post-Pliocene period. In some part, therefore, 

 of this chain, we may expect to discover tertiary plutonic rocks laid open 

 to view. What we already know of the structure of the Chilian Andes 

 seems to realize this expectation. In a transverse section, examined by 

 Mr. Darwin, between Valparaiso and Mendoza, the Cordillera was found 

 to consist of two separate and parallel chains, formed of sedimentary 

 rocks of different ages, the strata in both resting on plutonic rocks, by 

 which they have been altered. In the western or oldest range, called 

 the Peuquene-s, are black calcareous clay-slates, rising to the height of 

 nearly 14,000 feet above the sea, in which are shells of the genera Gri/- 

 pJisea, Turritella, Terehratula, and Ammonite. These rocks are sup- 

 posed to be of the age of the central parts of the secondary series of 

 Europe. They are penetrated and altered by dikes and mountain masses 

 of a plutonic rock, which has the texture of ordinary granite, but rarely 

 contains quartz, being a compound of albite and hornblende. 



The second or eastern chain consists chiefly of sandstones and con- 

 glomerates, of vast thickness, the materials of which are derived from 

 the ruins of the western chain. The pebbles of the conglomerates are, 

 for the most part, rounded fragments of the fossiliferous slates before 

 mentioned. The resemblance of the whole series to certain tertiary 

 deposits on the shores of the Pacific, not only in mineral character, but 

 in the imbedded lignite and silicified woods, leads to the conjecture that 

 they also are tertiary. Yet these strata are not only associated with trap 

 rocks and volcanic tuffs, but are also altered by a granite consisting of 

 quartz, felspar, and talc. They are traversed, moreover, by dikes of the 

 same granite, and by numerous veins of iron, copper, arsenic, silver, and 

 gold ; all of which can be traced to the underlying granite.* We have, 

 therefore, strong ground to presume that the plutonic rock, here exposed 

 on a large scale in the Chilian Andes, is of later date than certain terti' 

 ary formations. 



* Darwin, pp. 390, 406; second edition, p. 319. 

 37 



