This region has not been carefully explored, but
from the concurrent testimony of travellers, the granitic area is very
large: thus Von Eschwege gives a detailed section of these rocks,
stretching from Rio de Janeiro for 260 geographical miles inland in a
straight line; and I travelled for 150 miles in another direction, and saw
nothing but granitic rocks. Numerous specimens, collected along the whole
coast, from near Rio de Janeiro to the mouth of the Plata, a distance of
1,100 geographical miles, were examined by me, and they all belonged to
this class. Inland, along the whole northern bank of the Plata, I saw,
besides modern tertiary beds, only one small patch of slightly
metamorphosed rock, which alone could have formed a part of the original
capping of the granitic series. Turning to a well-known region, namely, to
the United States and Canada, as shown in Professor H.D. Rogers' beautiful
map, I have estimated the areas by cutting out and weighing the paper, and
I find that the metamorphic (excluding the "semi-metamorphic") and granite
rocks exceed, in the proportion of 19 to 12.5, the whole of the newer
Palaeozoic formations. In many regions the metamorphic and granite rocks
would be found much more widely extended than they appear to be, if all the
sedimentary beds were removed which rest unconformably on them, and which
could not have formed part of the original mantle under which they were
crystallised.
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