TO GEOLOGICAL RESEAECH. 67 



are sometimes aggregated in clusters and enclosed in sacculi. 

 These may be taken to represent the spores, while the sacculi 

 represent the sporangia, of gigantic Club-mosses of the Carboni- 

 ferous flora. 



Passing on now to the animal kingdom, we find that as early 

 as 1836, Ehrenberg had discovered that large rock-masses were 

 built up of the siliceous shells of minute organisms, classed by him 

 among the Infusoria, but now referred to the Alg?e under the 

 name of Diatoms, and since that time Mr. Sorby has done 

 good service by his investigation of limestones ; these he has 

 proved, for the most part, not to have originally possessed 

 any crystalline structure whatsoever, but to have been deposited as 

 mere mechanical aggregates of organic sands or clays, formed of 

 the debris of calcareous organisms of which the individuals can be 

 recognised. 



The comparison of the microscopic structure of the organisms 

 in chalk with those now found in the depths of the North Atlantic 

 Ocean, indicates that an immense deposit is now in course of 

 formation, quite analogous to what had previously taken place in 

 the seas of the Cretaceous period. A large proportion of the 

 North Atlantic sea-bed is found to be covered with an ooze, 

 chiefly formed by the shells of GlobigerincE, and this not a mere 

 surface-film, but an enormous mass, as proved by the large quan- 

 tities brought up by the dredge. 



Sir Wyville Thompson thus describes a sample of one and a- 

 half hundred-weight, obtained from a depth of nearly three miles : 

 " Under the microscope, the surface-layer was found to consist 

 chiefly of entire shells of Globigerina bulloides, large and small, 

 and of fragments of such shells mixed with a quantity of amor- 

 phous calcareous matter in fine particles, a little fine sand, and 

 many spicules, portions of spicules, and shells of RadioIa7'ia, a 

 few spicules of sponges, and a few frustules of diatoms. Below 

 the surface-layer, the sediment becomes gradually more compact ; 

 and a slight grey colour, due probably to the decomposing organic 

 matter, becomes more pronounced, while perfect shells of Globi- 

 gei-ina almost disappear, the fragments become smaller, and 

 calcareous mud, structureless, and in a fine state of division, is in 

 a greatly preponderating proportion. One can have no doubt, on 



