chap. xv. of the Cordillera. 589 



the southern parts of Copiapo, it must have been 

 mainly formed of clay-stone porphyries, with some 

 mica-schist, and with much sand-stone and jaspery 

 rocks exactly like the rocks in the gypseous formation, 

 and no doubt belonging to it's basal series. In several 

 other places also, during the accumulation of the 

 gypseous formation, its basal parts and the underlying 

 porphyritic conglomerate must likewise have been 

 already partially upheaved and exposed to wear and 

 tear ; near the Puente del Inca and at Coquimbo, there 

 must have existed masses of mica-schist or some such 

 rock, whence were derived the many small pebbles of 

 opaque quartz. It follows from these facts, that in 

 some parts of the Cordillera the upper beds of the 

 gypseous formation must lie unconformably on the 

 lower beds ; and the whole gypseous formation, in 

 parts, unconformably on the porphyritic conglomerate ; 

 although I saw no such cases, yet in many places the 

 gypseous formation is entirely absent ; and this, although 

 no doubt generally caused by quite subsequent denuda- 

 tion, may in others be due to the underlying porphy- 

 ritic conglomerate having been locally upheaved before 

 the deposition of the gypseous strata, and thus having 

 become the source of the pebbles of porphyry embedded 

 in them. In the porphyritic conglomerate formation, 

 in its lower and middle parts, there is very rarely any 

 evidence, with the exception of the small quartz pebbles 

 at Jajuel, near Aconcagua, and of the single pebble of 

 granite at Copiapo, of the existence of neighbouring 

 land : in the upper parts, however, and especially in the 

 district of Copiapo, the number of thoroughly well- 

 rounded pebbles of compact porphyries make me be- 

 lieve, that, as during the prolonged accumulation of 

 the gypseous formation the lower beds had already been 

 locally upheaved and exposed to wear and tear, so it 



