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The geological region passed over was also one of much interest, 
and from the observations of the gentlemen the following information 
has been derived. 
The geological structure, as far as their observations went, cor- 
responds to that of North Chili, with the exception of a narrow belt 
of sedimentary rock along the sea-coast, west of the granitic range, 
which is wanting in that country. This belt includes the island of 
San Lorenzo, and others, as well as the coast itself, to the extent of 
from seven to ten miles from the sea-beach. These sedimentary 
rocks are argillaceous, distinctly stratified, and more or less slaty, the 
layers being in many places discoloured by the red oxide of iron. In 
other places they appeared of a black colour, as if in the vicinity 
of coal beds, of which the existence was spoken of, but we did not 
discover any unequivocal traces of this substance. Some conspicuous 
examples of faults were noticed by Mr. Dana, along the coast of San 
Lorenzo. Many minerals were also found by this gentleman, among 
them gypsum was of frequent occurrence, also some fossils, for which 
information reference is made to the Geological Report. 
The hills and mountains to the eastward, joining the above sedi- 
mentary rocks, are exclusively of granite, which extends in width to 
the distance of forty-five geographical miles beyond Yaso. In places 
it has very much the appearance of a stratified rock, is much broken 
and variable in its character, so as to render it somewhat deceptive. 
Dr. Pickering observes, that this peculiar character or appearance is 
owing to the slow process of the decomposition of the rock in this dry 
climate, and which would in other places, subject to the ordinary 
fluctuations of seasons, be covered with several feet of earth. The 
same reasons will account for the duration of the Inca villages that 
cover many of the hills, and which a copious shower would entirely 
wash away. The gf anite on its eastern side was coarse-grained, pre- 
senting more of the ordinary appearance of that rock. 
Immediately eastward of the granite district commences the trap 
rocks, consisting for the most part of porphyry. Dr. Pickering traced 
the line of junction for some miles, the hills on one side being of 
granite, on the other porphyry. The eastern limit of the trap region 
is supposed to be distant some twenty miles from the western. The 
porphyry resembles the Swedish and that in the vicinity of Boston. 
Many porphyry pebbles, supposed to be of this formation, were 
found on the beach at Callao, having, it is to be presumed, been 
carried there by the action of the water-courses. 
