ANNIVERSARY ADDRESS OF THE PRESIDENT. IxXV 



from so short an examination. The mountains are steep, and abso- 

 lutely bare of vegetation ; the atmosphere is resplendently clear ; the 

 stratification distmct, and the rocks brightly and variously coloured : 

 some of the natural sections might be truly compared for distinctness 

 to those coloured in geological works*." The Peuquenes and the 

 Portillo in the one pass, and the Cumbre and Uspallata in the other, 

 are distinct parallel ranges, each differing considerably in composition 

 from the other. 



This part of the Cordillera consists of several parallel antichnal and 

 what Mr. Darwin calls "uniclinal" mountain-lines, ranging north and 

 south : in the main exterior lines, the strata are seldom inclined at a 

 high angle ; but in the central lofty ridges they are almost always 

 highly inclined, often vertical, and are broken by many great faults. 

 The strata that flank the chain are traversed by innumerable dykes of 

 igneous rocks, but these are rare in the central parts of the range. 



The lowest rock is a porphyritic claystone conglomerate, sometimes 

 between 6000 and 7000 feet thick. The imbedded fragments, which 

 vary in size from mere particles to eight inches in diameter, are both 

 angular and rounded, and both kinds occur in the same mass. 

 Sometimes the rock is a true conglomerate, at other times a breccia. 

 The fragments are claystone porphyry, a felspathic rock like altered 

 clayslate, and occasionally, but rarely, quartz. All the varieties of 

 conglomerate and breccia pass into each other ; and by metamorphic 

 action they are changed into porphyries, no longer retaining the least 

 trace of mechanical origin. The fragments Mr. Darwin supposes to 

 have been derived from miasses that were ejected from a submarine 

 crater, and those that are rounded he supposes to have been tritu- 

 rated in the heated and agitated water that filled the crater, from 

 something very analogous which he observed in some of the recent 

 volcanos of the Galapagos islands. 



Besides the porphyritic conglomerates and the well-characterized 

 metamorphic porphyry, there are other porphyries, which, though 

 differing slightly in composition, have had a distinct origin. They 

 contain large crystals of albite felspar and are often amygdaloidal, with 

 nodules of agate and calcareous spar. They occur in intrusive masses, 

 interstratified mth the conglomerate in several alternations, and have 

 all the appearance of submarine lavas, either forced in between the 

 planes of stratification of the conglomerate, or contemporaneous with 

 the deposit of the latter. Volcanic matter of sub-aerial origin is 

 everj^where rare in Chile, the few still active volcanos being confined 

 to the central and loftiest ranges. Metamorphic action has taken 

 place to a great extent ; it is seen in the gradual appearance of crystals 

 of felspar and epidote, in the blending of the imbedded fragments of 

 the porphyritic conglomerate, and in the disappearance of the planes 

 of stratification. 



Another variety of intrusive igneous rock, occurring in this part of 

 the Cordillera, is that which has been called Andesite, consisting 

 either of well-crystallized white albite, or soda-felspar, or of that 

 white mineral analysed by Abich and called by him Andesine, of 



* Page 176. 



