Ehrenberg on Infusoria in Igneous Rocks. 41 1 



and scoriae of different colours, and slightly consolidated. Each 

 successive saucer-shaped layer crops out all round the margin, form- 

 ing so many rings of various colours, and giving to the hill a fantastic 

 appearance. The outer ring is broad and of a white colour, hence it 

 resembles a course, round which horses have been exercised, and has 

 received the name of the Devil's Riding-School, by which it is most 

 generally known. These successive layers of ashes must have fallen 

 over the whole surrounding country ; but they have all been blown 

 away except in this one hollow, in which probably moisture accumu- 

 lated, either during an extraordinary year when rain fell, or during the 

 storms often accompanying volcanic eruptions. One of the layers of 

 a pinkish colour, and chiefly derived from small decomposed frag- 

 ments of pumice, is remarkable from containing numerous concre- 

 tions," &c. (Volcanic Islands, p. 47.) 



This singular example of volcanic ashes met with in a true vol- 

 canic island, insolated and situated off the coast of Africa, exhibits 

 however, on a careful microscopic investigation, none of the cha- 

 racters of an ordinary inorganic volcanic ash ; but, on the contrary, 

 the whole mass is of organic origin, scarcely changed in its separate 

 parts, but entirely deprived of every form of carbon, which has pro- 

 bably been dissipated on the mass being exposed to a red heat. 

 This completely rainless and treeless island, covered only with a 

 scanty vegetation, on which no land birds are able to exist, as we 

 are informed by Mr. Darwin in his ( Journal,' can hardly have had 

 such a periodical supply of water in this so-called f old volcano' as 

 to have allowed many plants to grow, since our traveller does not 

 mention the existence of their remains in that place. 



When it is considered that thirty species of organic bodies, chiefly 

 remains of plants (Phytolitharia), but including also siliceous-shelled 

 infusoria, have been obtained from this most characteristic form of 

 a tufaceous deposit in a circular band surrounding a supposed vol- 

 cano, the phenomenon appears beyond a doubt very enigmatical, 

 and requires to be considered in a somewhat new point of view in 

 order that it may be solved. 



The specimens of this tuff that were examined do not merely ex- 

 hibit the organic forms distributed more or less abundantly through 

 the mass, but they seem actually made up of them, since even the 



