42 LYELL'S ELEMENTS OF GEOLOGY. 



Slow Deposition of Strata. 



careous ; but Ehrenberg has discovered others in the flints of the 

 chalk, which, like the infusoria in tripoli, are siliceous. These 

 forms are especially apparent in the white coating of flints, often 

 accompanied by innumerable needle-shaped spicula? of sponges : 

 and the same are occasionally visible in the central parts of 

 chalk flints where they are of a lighter colour. After reflecting 

 on these discoveries, we are naturally led on to conjecture that, 

 as the formless cement in the semi-opal of Bilin has been de- 

 rived from the decomposition of animal remains, so also even 

 those parts of chalk flints in which no organic structure can be 

 recognized may nevertheless have constituted a part of micro- 

 scopic animalcules. 



" The dust we tread upon was once alive !" Byron. 



How faint an idea does this exclamation of the poet convey of 

 the real wonders of nature ! for here we discover proofs that the 

 calcareous and siliceous dust of which hills are composed has 

 not only been once alive, but almost every particle, albeit invisi- 

 ble to the naked eye, still retains the organic structure which, at 

 periods of time incalculably remote, was impressed upon it by 

 the powers of life. 



As 1 have dwelt upon the proofs of the slowness with which 

 fossiliferous strata in general have been produced, I may remark 

 that some writers have argued, from the appearances of certain 

 deposits containing coal, that sedimentary rocks of great thick- 

 ness have been accumulated with rapidity. This conclusion has 

 been drawn chiefly from a remarkable phenomenon, the posi- 

 tion of the trunks of fossil trees intersecting obliquely, and often 

 at right angles, the planes of many strata. For a full examina- 

 tion of this question, the reader is referred to the chapter on the 

 carboniferous formations, in the sequel ; and I shall merely say 

 here, that, although partial deposits have been thrown down in 

 the spots where these fossil trees occur in a comparatively short 

 lapse of time, yet we can by no means infer that a similar rate 

 of increase of carboniferous rocks prevailed simultaneously over 

 a wide area. On the other hand, the vegetable origin of coal is 

 now universally admitted by geologists ; and, when we discuss 

 the probable manner in which the terrestrial plants from which 

 it was derived were imbedded in marine shale and sandstone, we 

 shall find it necessary to suppose a long succession of operations. 



Freshwater and marine fossils. Strata, whether deposited 

 in salt or fresh ivater, have the same forms ; but the fossils are 

 very different in the two cases, for the same reason that aquatic 

 animals which frequent lakes and rivers are distinct from those 

 inhabiting the sea. As an example of English strata character- 

 ized by freshwater fossils, I may point out a formation which 



