70 THE VOYAGE OF H.M.S. CHALLENGER. 



paper, Ehrenberg rejects the idea that this deposit is the residue of the vegetation of 

 the island. 1 



Darwin in his Voyage of a Naturalist modified his first explanation of this deposit, 

 and stated the results of Ehrenberg's examination. After mentioning that Ehrenberg 

 considers this siliceous matter to have been ejected in its present state from the 

 volcano, he states that the appearance of the layers has led him to believe that they 

 were deposited under water, and considering the extreme dryness of the climate, he 

 has been compelled to suppose that torrents of rain had probably accompanied some 

 great eruption, and that a temporary lake was thus formed in which the ashes were 

 laid down. Perhaps one might now be justified in supposing that the lake was not 

 temporary. Although it were so, we may be quite sure that at some earlier period 

 the climate and productions of Ascension were quite different from what they are 

 now. 



The specimens of white earth and the concretions from the Devil's Eiding School, 

 which we have examined, correspond with Darwin's macroscopic description, and, in 

 general, with what Ehrenberg said of their microscopic constitution. Amongst the 

 specimens we have studied three varieties occur ; two of these are concretionary, and 

 both pass into the third by insensible gradations. The common variety is a pul- 

 verulent earthy rock, soiling the fingers, and to the touch resembling mealy diatomace- 

 ous earth ; the colour is yellowish white, inclining to pink. This variety is associated 

 with the spherical concretions of which Darwin speaks ; these are embedded in the 

 mealy mass. The nodules we have examined are from 1 to 3 centimetres in diameter. 

 They are built up of concentric zones sometimes with radial fissures ; spherical coat- 

 ings easily peel off, but the central part is more compact. Two nodules are sometimes 

 joined ; in other cases they bear the marks of small depressions. Except for their 

 rather large size, analogies are not wanting with certain pisoliths or globular forms 

 sometimes assumed by volcanic ashes. These globules are not generally very coherent, 

 but the third variety differs in this respect. In it the concretions are more irregular, 

 assuming discoidal, cylindrical, even coral-like forms ; the surface alone is earthy, the 

 internal part being compact, and so hard that steel will hardly scratch it. All the 

 particles which make up the interior zones are strongly cemented, and coloured brown 

 by iron. We may add that some of these nodules bear a great resemblance to some 

 flint concretions of the chalk. A summary analysis showed that the material contained 

 about 87 per cent, of silica, and that the loss by heating was 6 per cent. 



The various forms of this siliceous substance have the same microscopic composi- 



1 Ehrenberg, Ueber eineu bedeutenden Infusorien haltenden vulkanischen Aschen Tuff (Pyrobiolith) auf der 

 Insel Ascension (Bcrichte d. k. Akad. d. Wiss. Berlin, 1845, p. 140). Taking account of the name (Pyrobiolith) which 

 Ehrenberg gives to the deposit, and the conclusions he expresses in his memoir on the infusorian volcanic tufas of the 

 Rhenan country (Joe. cit., Bd. vi. p. 133), it is evident that he considers these deposits as of internal origin, and brought 

 to their present position by eruptions. 



