476 Basal Strata of the Cordillera, paetii. 



be distinct from stratification. Occasionally tliis rock 

 is somewhat carious ; and at one spot, namely, at the 

 C. of Quillota, it had a brecciated structure. Near the 

 mines of Jajuel, in a thick stratum of this feldspathic, 

 porphyritic slate, there was a layer of hard, blackish, 

 siliceous, infusible, compact clay-slate, such as I saw 

 nowhere else : at the same place I was able to follow 

 for a considerable distance the junction between the slate 

 and the conformably underlying porphyritic conglo- 

 merate, and they certainly passed gradually into each 

 other. "Wherever these slaty feldspathic rocks abound, 

 greenstone seems common ; at the C. of Quillota a bed 

 of well-crystallised greenstone lay conformably in the 

 midst of the feldspathic slate, with the upper and lower 

 junctions passing insensibly into it. From this fact, 

 and from the frequently porphyritic condition of the 

 slate, I should perhaps have considered this rock as an 

 erupted one (like certain laminated feldspathic lavas 

 in the trachytic series), had I not seen in T. del Fuego 

 how readily true clay-slate becomes feldspathic and 

 porphyritic, and had I not seen at Jajuel the included 

 layer of black, siliceous clay-slate, which no one could 

 have thought of igneous origin. The gentle passage 

 of the feldspathic slate, at Jajuel, into the porphyritic 

 conglomerate, which is certainly of aqueous origin, 

 should also be taken into account. 



The alternating strata of porphyries and porphyritic 

 conglomerate, and with the occasionally included beds 

 of feldspathic slate, together make a grand formation ; 

 in several places within the Cordillera I estimated its 

 thickness at from 6,000 to 7,000 feet. It extends for 

 manv hundred miles, forming the western flank of the 

 Chilian Cordillera ; and even at Iquique in Peru, 850 

 miles north of the southernmost point examined by me 

 in Chile, the coast-escarpment, which rises to a height 



