XXI 
BEDS OF INFUSORIA 
525 
secondly, that the still fluid lava within was packed by the 
centrifugal force, generated by the revolving of the bomb, against 
the external cooled 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 scoriae. 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 organised ; 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. From the absence of all carbonaceous matter, 
Professor Ehrenberg believes that these organic bodies have 
passed through the volcanic fire, and have been erupted in the 
state in which we now see them. The appearance of the layers 
induced me to believe that they had been deposited under water, 
though from the extreme dryness of the climate I was forced 
to imagine that torrents of rain had probably fallen during some 
great eruption, and that thus a temporary lake had been formed, 
into which the ashes fell. But it may now be suspected that 
the lake was not a temporary one. Anyhow we may feel sure 
that at some former epoch the climate and productions of 
Ascension were very different from what they now are. 
Where on the face of the earth can we find a spot on which 
close investigation will not discover signs of that endless cycle 
of change, to which this earth has been, is, and will be 
subjected ? 
On leaving Ascension we sailed for Bahia, on the coast of 
1 Monats. der Konig, Akad .. d. Wiss. zu Berlin. Vom April 1845. 
