Physical Structure and older stratified Deposits of Devonshire. 699 



portion of the old red sandstone. On the north-western coast of the county, 

 we were preceded by Mr. De la Beche, and we must refer our readers to the 

 many excellent details of his " Report." A few of our fossils from both coasts 

 will appear in the appended lists ; and, imperfect as they are, they appear to 

 sanction our "general views. The Petherwin, and we think also the highly 

 crystalline Tintagel slates, appear to be of the age of the Barnstaple slates, 

 and are therefore in the upper part of the old red sandstone. Such is a ge- 

 neral summary of the evidence on which our proposed classification is founded; 

 and the result is given in our accompanying map. (PI. L.) 



Concluding Remarks, &c. 



Having stated the grounds on which we have endeavoured to establish the 

 classification of the stratified rocks of Devonshire and Cornwall, it may be 

 well, before we quit the subject, to notice certain objections that have been 

 urged against our views. 



1. In the first place, our evidence may be considered defective, from the 

 absence of the carboniferous limestone, which in many parts of the British 

 Isles has so striking a position between the coal measures and the old red 

 sandstone. To this we might reply, by pointing out several instances in the 

 central counties of England, where the carboniferous system rests on the 

 lower formations, without the intervention of any bands of limestone. We 

 would, however, rather appeal to a more general fact — that the mountain lime- 

 stone, after forming in the north of England a great succession of beds, and 

 rising into mountains of considerable elevation, gradually thins out in its pro- 

 longation towards the south ; so that in South Wales it occupies but one.well- 

 defined band, and at length so entirely dies away in the western extremity of 

 Pembrokeshire, that the overlying culm measures and hard grits rest, in one 

 district, immediately on older rocks, without the intervention of any calcareous 

 matter whatsoever. With such a fact before us, we have no reason to be 

 surprised at the absence of the mountain limestone, where the culm measures 

 reappear in Devonshire and Cornwall. 



Considering the enormous thickness of the culm measures of these coun- 

 ties, it is by no means improbable that the mountain limestone is there repre- 

 sented by contemporaneous deposits of a different mineral type. For surely 

 it is now needless for us to prove that rocks, identical in geological position, 

 may entirely change their lithological structure, as they pass from one county 

 to another. A change of structure, however, implies a change of conditions 

 during the time of the deposit; and such a change might greatly modify the 

 forms of animal life. In this way we might, perhaps, explain the absence of 



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