486 Section by the Portillo Pass. paut ii. 



at [F] the gypseous formation. Its thickness is very 

 great. It consists in most parts of snow-white, hard, 

 compact gypsum, which breaks with a saccharine frac- 

 ture, having translucent edges ; under the blowpipe 

 gives out much vapour; it frequently includes nests 

 and exceedingly thin layers of crystallised, blackish 

 carbonate of lime. Large, irregularly shaped concre- 

 tions (externally still exhibiting lines of aqueous 

 deposition) of blackish-gray, but sometimes white, 

 coarsely and brilliantly crystallised, hard anhydrite, 

 abound within the common gypsum. Hillocks, formed 

 of the hardest and purest varieties of the white gypsum, 

 stand up above the surrounding parts, and have their 

 surfaces cracked and marked, just like newly baked 

 bread. There is much pale brown, soft, argillaceous 

 gypsum ; and there were some intercalated green beds 

 which I had not time to reach. I saw only one frag- 

 ment of selenite or transparent gypsum, and that 

 perhaps may have come from some subsequently formed 

 vein. From the mineralogical characters here given, 

 it is probable that these gypseous beds have undergone 

 some metamorphic action. The strata are much hidden 

 by detritus, but they appeared in most parts to be 

 highly inclined; and in an adjoining lofty pinnacle they 

 could be distinctly seen bending up, and becoming 

 vertical, conformably with the underlying porphyritio 

 conglomerate. In very many parts of the great moun- 

 tain-face [F], composed of thin gypseous beds, there 

 were innumerable masses, irregularly shaped and not 

 like dikes, yet with well-defined edges, of an imperfectly 

 granular, pale greenish or yellowish-white rock, essen- 

 tially composed of feldspar, with a little chlorite or 

 hornblende, epidote, iron-pyrites, and ferruginous pow- 

 der : I believe that these curious trappean masses have 

 been injected from the not far distant mountain-mass 



