568 Valley of Copiapo : paet n. 



formation [M M], which in one part encases, as repre- 

 sented in the coloured section, the foot of the andesitic 

 axis [L], of the already described fifth line, and in 

 another part entirely conceals it : in this latter case, 

 the gypseous or cretaceo-oolitic strata falsely appeared 

 to dip under the porphyritic conglomerate of the fifth 

 axis. The lowest bed of the gypseous formation, as 

 seen here pi], is of yellowish siliceous sandstone, pre- 

 cisely like that of Amolanas, interlaced in parts with 

 veins of gypsum, and including layers of the black, 

 calcareous, non-fissile slate-rock : the Turritella Andii, 

 Peden Dufreynoyi, Terebratula cenigma, var. and 

 some Gryphites were embedded in these layers. The 

 sandstone varies in thickness from only twenty to eighty 

 feet ; and this variation is caused by the inequalities in 

 the upper surface of an underlying stream of purple 

 clay-stone porphyry. Hence the above fossils here lie 

 at the very base of the gypseous or cretaceo-oolitic 

 formation, and hence they were probably once covered 

 up by strata about 7,000 feet in thickness : it is, how- 

 ever, possible, though from the nature of all the other 

 sections in this district not probable, that the por- 

 phyritic clay-stone lava may in this case have invaded a 

 higher level in the series. Above the sandstone there 

 is a considerable mass of much indurated, purplish- 

 black, calcareous clay-stone, allied in nature to the 

 often-mentioned black calcareous slate-rock. 



Eastward of the broad andesitic axis of this sixth 

 Hue, and penetrated by many dikes from it, there is 

 a great formation [P] of mica-schist, with its usual 

 variations, and passing in one part into a ferruginous 

 quartz-rock. The folio are curved and highly inclined, 

 generally dipping eastward. It is probable that this 

 mica-schist is an old formation, connected with the 

 granitic rocks and metamorphic schists near the coast ; 



