Page:Darwin Journal of Researches.djvu/516

This page has been validated.
494
[CHAP. XXI.
ASCENSION.

Journal of Researches (1860) Page 494.jpg

crust, and so produced the solid shell of stone; and lastly, that the centrifugal force, by relieving the pressure in the more central parts of the bomb, allowed the heated vapours to expand their cells, thus forming the coarsely cellular mass of the centre.

A hill, formed of the older series of volcanic rocks, and which has been incorrectly considered as the crater of a volcano, is remarkable from its broad, slightly hollowed, and circular summit having been filled up with many successive layers of ashes and fine scoriæ. These saucer-shaped layers crop out on the margin, forming perfect rings of many different colours, giving to the summit a most fantastic appearance; one of these rings is white and broad, and resembles a course round which horses have been exercised; hence the hill has been called the Devil's Riding School. I brought away specimens of one of the tufaceous layers of a pinkish colour; and it is a most extraordinary fact, that Professor Ehrenberg[1] finds it almost wholly composed of matter which has been organized: he detects in it some siliceous-shielded, fresh-water infusoria, and no less than twenty-five different kinds of the siliceous tissue of plants, chiefly of grasses.

  1. Monats. der König. Akad. d. Wiss. zu Berlin. Vom April, 1845.