286 LYELL'S ELEMENTS OF GEOLOGY. 



Plutonic Rocks of the Chalk, Oolite, and Lias. 



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

 rocks, by which they have been altered. In the western or old- 

 est 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 Gryphsea, Turritella, 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 horn- 

 blende. 



The second or eastern chain consists chiefly of sandstones 

 and conglomerates, 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 hg- 

 nite and silicified wood, 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 newer 

 than that of the western chain, and 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 underlymg 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 tertiary formations. 



Cretaceous period. — It was stated (p. 145.) that chalk as 

 well as lias have been altered by granite in the eastern Pyre- 

 nees. Whether such granite be cretaceous or tertiary cannot 

 easily be decided. 



Suppose ft, r, d, to be three 

 Fig. 289. members of the Cretaceous series, 



the lowest of which, b, has been 

 altered by the granite A, the modi- 

 fying influence not having extended 

 'so far as c, or having but slightly 

 affected its lowest beds. Now it 

 can rarely be possible for the ge- 

 ologist to decide whether the beds 

 d existed at the time of the intrusion of A, and alteration of b 

 and c, or whether they were subsequently thrown down upon c. 



* Darwin, pp. 390. 406. 



