370 Tertiary Formations, part h. 



CHAPTER XII. - 



ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND 



CHILE. 



Bio Negro — 8. Josef — Port Desire, white pumicemis mudstone with in- 

 fusoria — Port 8. Julian — Santa Cruz,basaltic lava of — P. Gallegos 

 — Eastern Tierra del Fuego ; leaves of extinct beech-trees — Sum- 

 mary on the Patagonian tertiary formations — Tertiary formations 

 of the Western Coast — Chonos and Chiloe groups, volcanic rocks of 

 — Coneep&um — Xavidad — Coquimbo — Summary — Age of the ter- 

 tiary formations — Lines of elevation — Silicified wood — Comparative 

 ranges of the extinct and living Mollusca on the West Coast of S. 

 America — Climate of the tertiary period — On the causes of the 

 absence of recent conchiferous deposits on the coasts of S. America 

 ■ — On the contemporaneous deposition and preservation of sedi- 

 mentary formations. 



Rio Negro. — I can add little to the details given by 

 M. d'Orbigny l on the sandstone formation of this dis- 

 trict. The cliffs to the south of the river are about 

 200 feet in height, and are composed of sandstone of 

 various tints and degrees of hardness. One layer, which 

 thinned out at both ends, consisted of earthy matter, of 

 a pale reddish colour, with some gypsum, and very like 

 (I speak after comparison of the specimens brought 

 home) Pampean mud : above this was a layer of compact 

 marly rock with dendritic manganese. Many blocks of 

 a conglomerate of pumice-pebbles embedded in hard 

 sandstone were strewed at the foot of the cliff, and had 

 evidently fallen from above. A few miles NE. of the 

 1 ' Voyage, Part. Geolog.' pp. 57-G5. 



