Geological observations on the volcanic islands and parts of South America visited during the voyage of H.M.S. 'Beagle' / by Charles Darwin.

  • Charles Darwin
Date:
1876
Catalogue details

Licence: Public Domain Mark

Credit: Geological observations on the volcanic islands and parts of South America visited during the voyage of H.M.S. 'Beagle' / by Charles Darwin. Source: Wellcome Collection.

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    the sea, measured from the upper hue of junction with the superincumbent basaltic lava, is about sixty feet ; and its thickness, although varying much from the inequalities of the underlying formation, may he esti- mated at about twenty feet. It consists of quite white calcareous matter, partly composed of organic debris, and partly of a substance which may he aptly compared in appearance with mortar. Fragments of rock and pebbles are scattered throughout this bed, often forming, especially in the lower part, a conglomerate. Many of the fragments of rock are whitewashed with a thin coating of calcareous matter. At Quail Island, the calcareous deposit is replaced in its lowest part by a soft, brown, earthy tuff, full of Turritellae ; this is • covered by a bed of pebbles, passing into sandstone, and mixed with fragments of echini, claws of crabs, and shells; the oyster shells still adhering to the rock on which they grew. Numerous white balls appearing like pisolitic concretions, from the size of a walnut to that of an apple, are embedded in this deposit; they usually have a small pebble in their centres. Although so like concretions, a close examination convinced me that they were Nulliporae, retaining their proper forms, but with their surfaces slightly abraded: these bodies (plants as they are now generally considered to be) exhibit under a microscope of ordinary power, no traces of organisation in their internal structure. Mr. Greorge E. Sowerby has been so good as to examine the shells which I col- lected : there are fourteen species in a sufficiently perfect condition for their characters to he made out with some degree of certainty, and four which can be referred only to their genera. Of the fourteen shells, of which a list is given in the Appendix, eleven are recent species ; one, though undescribed, is perhaps identical with a species which I found living in the harbour of Porto Praya ;
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