Geological Soctety. 157 
had a great elevation anterior to the tertiary period: and he thinks 
that a rough approximation may be made to the date of the com- 
mencement of the volcanic period, by observing the first associa- 
tion of streams of lava, with certain tertiary groups on the Patagonian 
side. 
A considerable portion of the extracts was devoted to a descrip- 
tion of the great tertiary groups on both sides of the chain of the Andes. 
Some of the details respecting the eastern side were derived from 
observations made on the Rio Negro, and on the line of a transverse 
section from Rio Santa Cruz to the base of the Cordilleras. These 
exhibit the structure of what Mr. Darwin calls the great southern ter- 
tiary formations of Patagonia. 
The lowest of these formations appears to be of great extent and 
thickness, and in one instance was found to contain a bed of an- 
cient lava, which seemed to mark the commencement of the eruption 
from the craters of the great chain of the Andes. It is character- 
ized by a great Oyster, and by other shells and corals, some of which 
belong to species now living on the neighbouring coasts. Over it is a 
deposit which Mr. Darwin describes as chiefly composed of rolled por- 
phyry pebbles, which he had himself traced for more than 700 miles. 
Overlying all the rest, and at a greater elevation above the level of 
the sea, were beds of recent shells, identical in species with the lit- 
toral shells of the neighbouring shores. Among these, he more espe- 
cially notices a widely extended bed of Muscles, which still retain 
their blue colour, and emit an animal smell when thrown in the fire. 
From these facts, he thinks the tertiary deposits of Patagonia may be 
separated into distinct periods, somewhat similar to those derived by 
Mr. Lyell from a comparison of the newer deposits of Europe: and 
in making the transverse section, he thought that he saw traces in the 
valley of Santa Cruz of an ancient channel, which must have traversed 
a great portion of the south part of the continent before the elevation 
of the tertiary groups. 
In noticing the groups on the western side of the Andes, he de- 
scribes an old tertiary deposit (eocine or miocene ?) south of Rio 
Maypo, and abundance of recent shells 1300 feet above the same level. 
He also describes the association of lava with beds containing recent 
shells in the island of Chiloe. Among other facts, he notices the ap- 
pearance of pitchstone among the beds of lava, and the occurrence of 
a forest growing over a bed of recent oysters 350 feet above the ac- 
tual level of the sea. All these recent shells are the littoral shells of 
the neighbouring shores ; from which he concludes that the elevation 
must have been gradual, or by successive hitches, similar to those by 
which the coast of Chili and, more recently, the coast of Chiloe have 
been unquestionably elevated. 
In addition to these very remarkable notices, Mr. Darwin mentions 
other fertiary deposits at Chiloe and Conception, composed of beds 
of sandstone and carbonaceous shale without shells, but containing 
many silicified trunks of dicotyledonous trees, and alternating with 
beds of lava. 
During the progress of the four years’ survey (in addition to the 
