5 $6 Valley of Copiapo. pake n. 



so strongly developed that the whole mass consists of 

 balls. I will not attempt to estimate the thickness of 

 the strata in the gypseous formation hitherto described, 

 but it must certainly be very many hundred feet. Bed 

 (11) is at least 800 feet in thickness: it consists of thin 

 layers of whitish, greenish, or more commonly brown, 

 fine-grained indurated tuffs, which crumble into angular 

 fragments : some of the layers are semi-porcellanic, 

 many of them highly ferruginous, and some are almost 

 composed of carbonate of lime and iron with drusy 

 cavities lined with quartz-crystals. Bed (12), dull pur- 

 plish or greenish or dark-gray, very compact and much 

 indurated mudstone : estimated at 1,500 feet in thick- 

 ness : in some parts this rock assumes the character of 

 an imperfect coarse clay-slate ; but viewed under a lens, 

 the basis always has a mottled appearance, with the 

 edges of the minute component particles blending to- 

 gether. Parts are calcareous, and there are numerous 

 veins of highly crystalline carbonate of lime charged 

 with iron. The mass has a nodular structure, and is 

 divided by only a few planes of stratification : there 

 are. however, two lavers, each about eighteen inches 

 thick, of a dark brown, finer-grained stone, having a 

 conchoidal, semi-porcellanic fracture, which can be 

 followed with the eye for some miles across the country. 

 I believe this last great bed is covered by other 

 nearly similar alternations ; but the section is here 

 obscured by a tilt from the next porphyritic chain, 

 presently to be described. I have given this section in 

 detail, as being illustrative of the general character of 

 the mountains in this neighbourhood ; but it must not 

 be supposed that any one stratum long preserves the 

 same character. At a distance of between only two and 

 three miles, the green mudstones and white -indurated 

 tuffs are to a great extent replaced by red sandstone 



