Ch. XXXIV.] PLUTONIC ROCKS IN THE ANDES. 721 



IV. are the most remote from each other in position, although of 

 contemporaneous date. According to this hypothesis, the convul- 

 sions of many periods will he required before Recent or Post-tertiary 

 granite will he upraised so as to form the highest ridges and central 

 axes of mountain-chains. During that time the Recent strata No. 4 

 might he covered by a great many newer sedimentary formations. 



Eocene Granite and Plutonic Rocks. — In a former part of this vol- 

 ume (p. 307), the great nummulitic formation of the Alps and P3Te- 

 nees was 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 

 elate breaking out in the central axis or most disturbed region of the 

 loftiest chain in Europe. Accordingly, in the Swiss Alps, even the 

 flysch, or upper portion of the nummulitic series, has been occasion- 

 ally invaded by plutonic rocks, and converted into crystalline schists 

 of the hypogene class. There can be little doubt that even the 

 talcose granite or gneiss of Mont Blanc itself has been in a fused or 

 pasty state since the flysch was deposited at the bottom of the sea ; 

 and the question as to its age is not so much whether it be a second- 

 ary or tertiary granite or gneiss, as whether it should be assigned to 

 the Eocene or Miocene epoch. 



Great upheaving movements have been experienced in the region 

 of the Andes, during the Post-tertiary period. In some part, there- 

 fore, 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 sec- 

 tion,- 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 rest- 

 ing on plutonic rocks, by which they have been altered. In the west- 

 ern or oldest range, called the Peuquenes, are black calcareous clay- 

 slates, rising to the height of nearly 14,000 feet above the sea, in 

 which are shells of the genera Gryphcea, Turritelhi, Terebratula, and 

 Ammonite. These rocks are supposed 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 conjec- 

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