86 GEOLOGICAL MEMOIRS. 



ed, owing to their toughness. The sandstone from Engers appears to 

 have been erupted in the condition of fused lumps, which have after- 

 wards been united into a mass by a cement of a very different kind*. 



II. On a remarkable Tuff of Volcanic Ashes containing Infusoria^ 

 from the Island of Ascension. 



The author was indebted to Mr. Darwin for several highly in- 

 teresting specimens for microscopic investigation, obtained during 

 his journey ; and amongst them was a singular white and soft vol- 

 canic tuff obtained from an extinct volcano in the island of Ascen- 

 sion. Before stating the result of his examination of this rock, the 

 author quotes from Mr. Darwin's work the following account of the 

 circumstances of its occurrence : — 



" Concretions hipumiceous tuff. — The hill marked in the map ' Crater of an old 

 volcano/ has no claims to this appellation which I could discover, except in being 

 surmounted by a circular, very shallow, saucer-like summit, nearly half a mile in 

 diameter. This hollow has been nearly filled up with many successive sheets of 

 ashes and scoriae of diflferent colours, and slightly consohdated. Each successive 

 saucer-shaped layer crops out all round the margin, forming so many rings of va- 

 rious colours, and giving to the hill a fantastic appearance. The outer ring is 

 broad and of a white colour, hence it I'esembles 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 accumulated, either during an ex- 

 traordinaiy year when rain fell, or during the storms often accompanying volca- 

 nic eruptions. One of the layers of a pinkish colour, and chiefly derived from 

 small decomposed fragments of pumice, is remarkable from containing numerous 

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



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

 canic island, isolated 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 lieat. 

 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 ' 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 phsenomenon 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 author here appends a table, in which the occurrence of each species in 

 each one of the thirty-nine different rocks of the Hochsimmer section is recorded. 

 It will be found facing page 139 of the Proceedings of the Berlin Academy (i?e- 

 ricUe) for 1845. 



