chap. xii. Tier r a del Fuego. 385 



of S. Cruz), I am informed by Captain Sulivan, R.N., 

 that there is a gravel-capped plain from 200 to 300 feet 

 in height, formed of numerous strata, some fine-grained 

 and pale-coloured, like the upper beds at the mouth of 

 the S. Cruz, others rather darker and coarser, so as to 

 resemble gritstones or tuffs ; these latter include rather 

 large fragments of apparently decomposed volcanic 

 rocks ; there are, also, included layers of gravel. This 

 formation is highly remarkable, from abounding with 

 mammiferous remains, which have not as yet been 

 examined by Professor Owen, but which include some 

 large, but mostly small, species of Pachydermata, 

 Edentata, and Rodentia. From the appearance of the 

 pale-coloured, fine-grained beds, I was inclined to 

 believe that they corresponded with the upper beds of 

 the S. Cruz; but Professor Ehrenberg, who has examined 

 some of the specimens, informs me that the included 

 microscopical organisms are wholly different, being 

 fresh and brackish water forms. Hence the 200 to 

 300 feet plain at Port Gallegos is of unknown age, but 

 probably of subsequent origin to the great Patagonian 

 tertiary formation. 



Eastern Tierra del Fuego. — Judging from the 

 height, the general appearance, and the white colour 

 of the patches visible on the hill sides, the uppermost 

 plain, both on the north and western side of the Strait 

 of Magellan, and along the eastern coast of Tierra del 

 Fuego as far south as near Port St. Polycarp, probably 

 belongs to the great Patagonian tertiary formation. 

 These higher table-ranges are fringed by low, irregular, 

 extensive plains, belonging to the boulder formation, 1 

 and composed of coarse unstratified masses, sometimes 

 associated (as north of C. Virgin's) with fine, laminated, 

 muddy sandstones. The cliffs in Sebastian Bay are 



1 Described in the ' Geological Transactions,' vol. vi. p. 415. 



