43 6 Patagonia : paet n. 



deposits appear to be present. At Cape Blanco, there 

 is quartz rock, very like that of the Falkland Islands, 

 and some hard^ blue, siliceous clay-slate. 



At Port Desire there is an extensive formation of 

 the claystone porphyry, stretching at least twenty-five 

 miles into the interior : it has been denuded and deeply 

 worn into gullies before being covered up by the ter- 

 tiary deposits, through which it here and there projects 

 in hills : those north of the bay beino- 440 feet in 

 height. The strata have in several places been tilted 

 at small angles, generally either to XX W. or SSE. 

 By gradual passages and alternations, the porphyries 

 change incessantly in nature. I will describe only 

 some of the principal mineralogical changes, which are 

 highly instructive, and which I carefully examined. 

 The prevailing rock has a compact purplish base, with 

 crystals of earthy or opaque feldspar, and often with grains 

 of quartz. There are other varieties, with an almost truly 

 trachytic base, full of little angular vesicles and crystals 

 of glassy feldspar ; and there are beds of black perfect 

 pitchstone, as well as of a concretionary imperfect 

 variety. On a casual inspection, the whole series would 

 be thought to be of the same plutonic or volcanic 

 nature with the trachytic varieties and pitchstone ; but 

 this is far from being the case, as much of the porphyry 

 is certainly of metamorphic origin. Besides the true 

 porphyries, there are many beds of earthy, quite white 

 or yellowish, friable, easily fusible matter, resembling 

 chalk, which under the microscope is seen to consist of 

 minute broken crystals, and which, as remarked in a 

 former chapter, singularly resembles the upper tufaceous 

 beds of the Patagonian tertiary formation. This earthy 

 substance often becomes coarser, and contains minute 

 rounded fragments of porphp'ies and rounded grains of 

 quartz, and in one case so many of the latter as to 



