THE GEOLOGICAL RECORD. 143 



there are chemical agencies which work without any 

 compassion for what is fine and delicate, and by these 

 we find great thicknesses of rock apparently stripped 

 of their fossils. Where the whole stratum consists of 

 remains of once living organisms, as in seams of coal, 

 it has been shown that we have no reason to suppose 

 that any complete or adequate memorials are left us of 

 the whole vegetation of any particular period or any 

 particular area; since Dr. Lindley has found, by actual 

 experiment, that diiferent vegetables have very different 

 powers of resisting decay, and that pines and ferns and 

 lycopodia will be well preserved after long immersion in 

 water, while the same treatment causes the disappearance 

 of grasses and sedges, of the oak-tree and the ash 1 . 



Even those rocks which preserve fossils most carefully 

 may themselves be crumbled to pieces, fossils and all, 

 by the process of denudation. 



Denudation is the laying bare of one stratum, or por- 

 tion of a stratum , by the removal of another. It is 

 carried on principally by rains and rivers and the action 

 of the sea-waves upon the sea-border. To the last- 

 mentioned agency the geologist is highly indebted ; to 

 the others also he owes a debt : but consider how they 

 all do their work. Much of the material dealt with they 

 pound into mud or sand, and in these any fragments 

 that escape the trituration are, sooner or later, again 

 buried. They may tear open the rocks, and expose for 

 a brief period the most interesting and unique fossils; 



1 See 'Lecture on Coal/ by W. Boyd Dawkina, Esq., M.A., F.R.S. 



Manchester, 1870. 



