Chapter 5

ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.

Rio Negro.
S. Josef.
Port Desire, white pumiceous mudstone with Infusoria.
Port S. Julian.
Santa Cruz, basaltic lava of.
P. Gallegos.
Eastern Tierra del Fuego; leaves of extinct beech-trees.
Summary on the Patagonian tertiary formations.
Tertiary formations of the Western Coast.
Chonos and Chiloe groups, volcanic rocks of.
Concepcion.
Navidad.
Coquimbo.
Summary.
Age of the tertiary 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 coast of S. America.
On the contemporaneous deposition and preservation of sedimentary formations.

RIO NEGRO.

I can add little to the details given by M. d'Orbigny on the sandstone formation of this district. ("Voyage" Part Geolog. pages 57-65.) The cliffs to the south of the river are about two hundred 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 N.E. of the town, I found, low down in the sandstone, a bed, a few inches in thickness, of a white, friable, harsh-feeling sediment, which adheres to the tongue, is of easy fusibility, and of little specific gravity; examined under the microscope, it is seen to be pumiceous tuff, formed of broken transparent crystals. In the cliffs south of the river, there is, also, a thin layer of nearly similar nature, but finer grained, and not so white; it might easily have been mistaken for a calcareous tuff, but it contains no lime: this substance precisely resembles a most widely extended and thick formation in Southern Patagonia, hereafter to be described, and which is remarkable for being partially formed of infusoria. These beds, conjointly with the conglomerate of pumice, are interesting, as showing the nature of the volcanic action in the Cordillera during this old tertiary period.

In a bed at the base of the southern cliffs, M. d'Orbigny found two extinct fresh-water shells, namely, a Unio and Chilina. This bed rested on one with bones of an extinct rodent, namely, the Megamys Patagoniensis; and this again on another with extinct marine shells. The species found by M. d'Orbigny in different parts of this formation consist of:--

1. Ostrea Patagonica, d'Orbigny, "Voyage, Pal." (also