At the mouth of the valley of Coquimbo, the surface consists wholly of gravel, and stands from 300 to 350 feet above the level of the sea, being about one hundred feet higher than in other parts.In these other and lower parts the superficial beds consist of calcareous matter, and rest on ancient tertiary deposits hereafter to be described.The uppermost calcareous layer is cream-coloured, compact, smooth-fractured, sub-stalactiform, and contains some sand, earthy matter, and recent shells.It lies on, and sends wedge-like veins into, a much more friable, calcareous, tuff-like variety; and both rest on a mass about twenty feet in thickness, formed of fragments of recent shells, with a few whole ones, and with small pebbles firmly cemented together.(In many respects this upper hard, and the underlying more friable, varieties, resemble the great superficial beds at King George's Sound in Australia, which I have described in my "Geological Observations on Volcanic Islands." There could be little doubt that the upper layers there have been hardened by the action of rain on the friable, calcareous matter, and that the whole mass has originated in the decay of minutely comminuted sea-shells and corals.) This latter rock is called by the inhabitants losa, and is used for building: in many parts it is divided into strata, which dip at an angle of ten degrees seaward, and appear as if they had originally been heaped in successive layers (as may be seen on coral-reefs) on a steep beach.This stone is remarkable from being in parts entirely formed of empty, pellucid capsules or cells of calcareous matter, of the size of small seeds: a series of specimens unequivocally showed that all these capsules once contained minute rounded fragments of shells which have since been gradually dissolved by water percolating through the mass.(I have incidentally described this rock in the above work on Volcanic Islands.)The shells embedded in the calcareous beds forming the surface of this fringe-like plain, at the height of from 200 to 250 feet above the sea, consist of:--1.Venus opaca.
2.Mulinia Byronensis.
3.Pecten purpuratus.
4.Mesodesma donaciforme.
5.Turritella cingulata.
6.Monoceros costatum.
7.Concholepas Peruviana.
8.Trochus (common Valparaiso species).
9.Calyptraea Byronensis.
Although these species are all recent, and are all found in the neighbouring sea, yet I was particularly struck with the difference in the proportional numbers of the several species, and of those now cast up on the present beach.I found only one specimen of the Concholepas, and the Pecten was very rare, though both these shells are now the commonest kinds, with the exception, perhaps, of the Calyptraea radians, of which I did not find one in the calcareous beds.I will not pretend to determine how far this difference in the proportional numbers depends on the age of the deposit, and how far on the difference in nature between the present sandy beaches and the calcareous bottom, on which the embedded shells must have lived.
(DIAGRAM 8.--SECTION OF PLAIN OF COQUIMBO.
Section through Plain B-B and Ravine A.
Surface of plain 252 feet above sea.
A.Stratified sand, with recent shells in same proportions as on the beach, half filling up a ravine.
B.Surface of plain, with scattered shells in nearly same proportions as on the beach.
C.Upper calcareous bed, and D.Lower calcareous sandy bed (Losa), both with recent shells, but not in same proportions as on the beach.
E.Upper ferrugino-sandy old tertiary stratum, and F.Lower old tertiary stratum, both with all, or nearly all, extinct shells.)On the bare surface of the calcareous plain, or in a thin covering of sand, there were lying, at a height from 200 to 252 feet, many recent shells, which had a much fresher appearance than the embedded ones: fragments of the Concholepas, and of the common Mytilus, still retaining a tinge of its colour, were numerous, and altogether there was manifestly a closer approach in proportional numbers to those now lying on the beach.In a mass of stratified, slightly agglutinated sand, which in some places covers up the lower half of the seaward escarpment of the plain, the included shells appeared to be in exactly the same proportional numbers with those on the beach.On one side of a steep-sided ravine, cutting through the plain behind Herradura Bay, I observed a narrow strip of stratified sand, containing similar shells in similar proportional numbers; a section of the ravine is represented in Diagram 8, which serves also to show the general composition of the plain.I mention this case of the ravine chiefly because without the evidence of the marine shells in the sand, any one would have supposed that it had been hollowed out by ****** alluvial action.
The escarpment of the fringe-like plain, which stretches for eleven miles along the coast, is in some parts fronted by two or three narrow, step-formed terraces, one of which at Herradura Bay expands into a small plain.
Its surface was there formed of gravel, cemented together by calcareous matter; and out of it I extracted the following recent shells, which are in a more perfect condition than those from the upper plain:--1.Calyptraea radians.
2.Turritella cingulata.
3.Oliva Peruviana.
4.Murex labiosus, var.
5.Nassa (identical with a living species).
6.Solen Dombeiana.
7.Pecten purpuratus.
8.Venus Chilensis.
9.Amphidesma rugulosum.The small irregular wrinkles of the posterior part of this shell are rather stronger than in the recent specimens of this species from Coquimbo.(G.B.Sowerby.)10.Balanus (identical with living species).
