410 Geological Memoirs, 



accumulated in layers, such a mode of accounting for the phsenomena 

 would best agree with the result of the author's investigations, and 

 his knowledge of the materials of which the deposit is made up. 



10. With regard to the very puzzling question, whence these infu- 

 soria and Phytolitharia came, he suggests, partly on account of their 

 including forms not now met with in a living state, that they may 

 possibly have formed layers of turf, or perhaps brown coal, which 

 by some accident have come within the range of volcanic activity ; 

 and owing to their incombustible and almost infusible condition 

 have been erupted as showers of ashes. Such layers of infusoria he 

 has already shown to exist, forming and accompanying the fissile 

 coal near Siegburg and Geistingen on the Rhine, although on the 

 Upper Rhine none have yet been determined. The tuff and firestone 

 appear to have had a similar volcanic origin, but to be the result of 

 eruptions of mud in which the infusorial strata have not been repeat- 

 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 in pumiceous 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 shal- 

 low, saucer-like summit, nearly half a mile in diameter. This hol- 

 low bas been nearly filled up with many successive sheets of ashes 



* 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 (Berichte) for 1845. 



