THE GEOLOGY OF THE CIIINCHAS ISLANDS. 
177 
found also in the central depressed portions of the island. Beds of 
coarse gravel grit and of an argillaceous sandstone also occur, and 
decisive evidence of its marine nature is afforded in a rather exten¬ 
sive bed of fossilized shell breccia, which may be seen on the south¬ 
west side of the North Island. 
On examination, this latter locality gives the following section:— 
Commencing above, immediately beneath the guano, which is 
here of a very dark colour, and an average depth of six inches, we 
meet a bed of dark argillaceous sandstone ; next, a bed of coarse 
sandstone grit passing into a shell breccia, the breccia varying in 
thickness from 1 to 4 feet, and composed of numerous Balani, part 
broken, part quite perfect, and many in position, all unrolled, pre¬ 
serving their natural sharpness, so that their identification is quite 
easy. They closely approach Balanus tintinnabulum , which species is 
dredged in the adjoining seas, and in abundance covers the rocks and 
base of the cliffs beneath, but they do not appear to be quite identical 
with this species. Along with the Balani are found single valves 
and broken shells of an Ostrea (this mollusc is not now, as far as can 
be judged, found here), and a few scattered spines of apparently a 
Cidaris, the whole cemented together into a very compact mass. 
Next, a belt of very fine friable grit, intersected by white veins 
of anhydrous sulphate of lime, which passes gradually into a very fine 
argillaceous sandstone, of greenish-gray colour and compact texture, 
also intersected by the sulphate of lime veins, and in parts deeply 
stained with iron. In some places this latter bed overlies the shell 
breccia, and appears to be a continuation of the bed of dark sand¬ 
stone immediately underlying the guano. These sandstone beds 
reappear on the other side of the island, and are probably continuous 
under the guano. Beneath the sandstone lies the usual granite of the 
island. Not far from the locality described, a thick bed of very coarse 
conglomerate, apparently identical with that found on the summits 
of the cliffs elsewhere, separated the granite from the green sand¬ 
stone; the shell breccia bed is also much thicker. 
The greenish-gray sandstone crumbles much under the influence 
of the weather, the veins of white sulphate of lime, in that case, 
protruding as thin ledges, in some parts some inches wide. The 
shell breccia extends along the face of the cliff for some hundred 
yards in length, but I had no means of ascertaining its extent inland. 
In many places along this side of the island an ammoniacal incrus- 
Vol. VII. T 
