53/108 The varieties of porphyry thus produced are endless, but nowhere have I yet met with rocks which have flowed in a stream; dykes of greenstone are very numerous. Modern volcanic action is entirely shut up in the very central parts (which cannot now be reached on account of the snow) of the Cordilleras. In the south of the R.Maypu I examined the Tertiary plains, already partially described by M.Gay. "Rapport fait a l'Academie Royale des Sciences, sur les Travaux Geologiques de M.Gay," by Alex. |