[ 213 ] 



XXX. Note on the Differences^ either Original or consequeiit 

 onDistiirbance, "which are observable in the Secondary Stratified 

 Bocks. Bij Henry T. i)E la Beche, Esq. F.R.S. S)X.* 



"VrUMEROUS smaller variations in the mineralogical struc- 

 -'-^ ture of the secondary stratified rocks have been long ac- 

 knowledged and pointed out by many geologists. The greater 

 or less development of a limestone or sandstone formation, the 

 want of certain beds in a given series, the alteration of rocks 

 within short distances from masses or veins of trap, &c., have 

 for some time been remarked, and the greater or less import- 

 ance that should be attached to these circumstances, upon the 

 whole, fairl}^ appreciated : but the greater changes, such as 

 the substitution of dark compact limestones and sandstones 

 for the green-sand of England and the North of France, though 

 long since noticed by M. Alex. Brongniart ; the transforma- 

 tion of the whole oolite system into compact dark-coloured 

 limestones resembling those commonly called transition ; the 

 occasional change of all the limestones, from the chalk to those 

 in the red sandstone formation inclusive, into dolomite, more 

 or less crystalline according to circumstances, with other dif- 

 ferences on the great scale, — have not generally met with that 

 attention which the importance of the subject, in a geological 

 point of view, seems to require. 



This inattention has probably in a great measure arisen 

 from the value attached to the different mineralogical struc- 

 tures which, it was supposed, characterized rocks deposited at 

 different geological epochs. Thus all crystalline limestones were 

 considered primitive; all dark-coloured limestones, very com- 

 pact and with a certain mineralogical structure, transition; 

 and all sandstones, when of the necessary colour and hardness, 

 grauwacke : and when contrary opinions were advanced, there 

 was always supposed to be some error on the part of the ob- 

 server. It is true, many geologists did not admit this depend- 

 ence on mineralogical structure; but it is equally true, that the 

 greater number were in favour of it. 



Geology, perhaps more than any science, requires a com- 

 bination of observations ; it is only from an accumulation of 

 facts that any real progress can be made, and it is (]uite clear 

 that this re(juires the labours of the many. Fortunately, in tiie 

 j)resent day there is no want of those who continually contri- 

 bute to our stock of knowledge, more ])articularly in this quarter 

 of liu; world ; and we see that Euro})e, though no very large 

 poilion of the cartli'3 surface, is fruitlUl in examples of great 



• Communicated by the Author. 



differences 



