made
along its base to the north of Crato. I now began to inquire if
anything like chalk was found in the neighbourhood, when I learned
there were several pits in the Serra, whence the inhabitants obtained
it for the purpose of white-washing their houses; these pits I
afterwards found to be situated in a deep layer of red-coloured
diluvial clay, which hes immediately over the sandstone of the Serra.
In a ravine near Crato I endeavoured to ascertain the formation on
which the sandstone rested, when I found it to consist of several
layers of more or less compact limestones and marls, with a bed of
lignite about two feet thick j upon what these rested I could not at
that time ascertain, but some time afterwards when I crossed to the
west side of the range, I found these limestones existing upon a
deposit of very dark-red coarse-grained sandstone, abounding in small
nodules of iron-stone. Thus we find that the structure of the rocks in
this locality is very similar to that of the chalk formation in
England; there is
1st. A ferruginous sandstone deposit, equivalent to the lower green sand or Shanklin sand.
2nd. A deposit of marls, soft and compact limestones, and lignite, equivalent to the English gault.
3rd.
A very thick deposit of fine-grained, soft, variously coloured
sandstone, containing Ichthyolites, equivalent to the upper green sand
of England.
4th. The white chalk itself, and flints occurring in pits partially covered by red diluvial clay.
Elints
are very common along the foot of the Serra, to the N.W. of Crato, but
none were found in any of the chalk-pits that I examined: I learned,
however, that at a considerable distance to the north of Crato, at a
portion of this mountain range, called the Serra de Botarite, both
chalk and flints are far more abundant than they are near the former
place, where they seem to have been almost entirely washed away,
previous to the deposition of the red clay in which they are now found.
Since the time when these rocks were first deposited at the
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