536 Section from part il 



and porphyritic conglomerate of great thickness, dip- 

 ping at an average angle of 20° to NE. by N. The 

 uppermost beds consist of conglomerates and sandstone 

 only a little metamorphosed, and conformably covered 

 by a gypseous formation of very great thickness, but 

 much denuded. This gypseous formation, where first 

 met with, lies in a broad valley or basin, a little south- 

 ward of the mines of Los Hornos : the lower half alone 

 contains gypsum, not in great masses as in the Cordillera, 

 but in innumerable thin layers, seldom more than an 

 inch or two in thickness. The gypsum is either opaque 

 or transparent, and is associated with carbonate of lime. 

 The layers alternate with numerous varying ones of a 

 calcareous clay-shale (with strong aluminous odour, 

 adhering to the tongue, easily fusible into a pale green 

 glass), more or less indurated, either earthy and cream 

 coloured, or greenish and hard. The more indurated 

 varieties have a compact, homogeneous almost crystal- 

 line fracture, and contain granules of crystallised oxide 

 of iron. Some of the varieties almost resemble hone- 

 stones. There is also a little black, hardly fusible, 

 siliceo-calcareous clay-slate, like some of the varieties 

 alternating with gypsum on the Peuquenes range. 



The upper half of this gypseous formation is mainly 

 formed of the same calcareous clay-shale rock, but 

 without any gypsum, and varying extremely in nature : 

 it passes from a soft, coarse, earthy, ferruginous state, 

 including particles of quartz, into compact clay-stones 

 with crystallised oxide of iron, — into porcellanic layers, 

 alternating with seams of calcareous matter. — and into 

 green porcelain-jasper excessively hard, but easily 

 fusible. Strata of this nature alternate with much 

 black and brown siliceo-calcareous slate, remarkable 

 from the wonderful number of huge embedded logs of 

 silicified wood. This wood, according to Mr. R. Brown, 



