chap. xv. Porphyritic Formation, 559 



porphyry, but remarkable from containing one pebble 

 of granite ; — of which fact no instance has occurred in 

 the sections hitherto described. Above this conglome- 

 rate, there is a black siliceous clay-stone, and above it 

 numerous alternations of dark-purplish and green por- 

 phyries, which may be considered as the uppermost 

 limit of the porphyritic conglomerate formation. 



Above these porphyries comes a coarse, arenaceous 

 conglomerate, the lower half white and the upper half 

 of a pink colour, composed chiefly of pebbles of various 

 porphyries, but with some of red sandstone and jaspery 

 rocks. In some of the more arenaceous parts of the 

 conglomerate, there was an oblique or current lamina- 

 tion ; a circumstance which I did not elsewhere observe. 

 Above this conglomerate, there is a vast thickness of 

 thinly stratified, pale-yellowish, siliceous sandstone, 

 passing into a granular quartz-rock, used for grindstones 

 (hence the name of the place Las Amolanas), and 

 certainly belonging to the gypseous .formation, as does 

 probably the immediately underlying conglomerate. 

 In this yellowish sandstone there are layers of white 

 and pale-red siliceous conglomerate ; other layers with 

 small, well-rounded pebbles of white quartz, like the 

 bed at the R. Claro at Ooquimbo ; others of a greenish, 

 fine-grained, less siliceous stone, somewhat resembling 

 the pseudo-honestones lower down the valley ; and 

 lastly, others of a black calcareous shale-rock. In one 

 of the layers of conglomerate, there was embedded a 

 fragment of mica-slate, of which this is the first instance ; 

 hence, perhaps, it is from a formation of mica-slate, 

 that the numerous small pebbles of quartz, both here 

 and at Coquimbo, have been derived. Not only does 

 the siliceous sandstone include layers of the black, 

 thinly stratified, not fissile, calcareous shale-rock, but 

 in one place the whole mass, especially the upper part, 



