Physical Structure aud older stratified Deposits of Devonshire. 691 



to distrust the views, we had first entertained respecting the age of the older 

 stratified groups above described. For one of the authors_, during the sum- 

 mer of 1838, re-examined a part of the great transverse section of Devon- 

 shire, and was more than ever convinced that, with some ambiguous excep- 

 tions, there was on each side of the great trough a true passage from the 

 lower division of the culm-measures into the inferior fossiliferous group. 

 Thus the section from Marwood across the culm series to the calcareous 

 slates of South Petherwin seemed to be perfectly symmetrical (PI. LI, fig. 2.), 

 the lower beds of the culm-measures being repeated (as described in our pre- 

 vious paper) on both sides of the trough ; from beneath which emerge on the 

 respective sides, and in a regular uninterrupted sequence, the calcareous 

 slates of Petherwin and Barnstaple. Now, if this view be correct, it follows, 

 almost inevitably, from our own evidence, that the calcareous slates in question 

 must be very near the upper limit of the old red sandstone; and this con- 

 clusion being once admitted, we should have little difficulty in classing, not 

 merely the calcareous rocks of North Devon, but also the fossiliferous slates 

 of South Devon and Cornwall, with the old red sandstone. 



To the preceding facts, bearing on the classification of the older rocks of 

 Devon and Cornwall, we might add some remarks on the mineral structure 

 of the two counties — on the existence of red conglomerates among the beds of 

 the Cornish killas — and on the large tracts, both of North and South Devon, 

 filled with rocks identical in structure with the most ordinary forms of old red 

 sandstone. But we forbear to dwell on such facts as these, wishing to build 

 our present conclusions chiefly on zoological evidence. 



Guided then by the evidence already stated, and the conclusions to which 

 it seemed to point, we have re-examined tiie fossils obtained, either by oui 

 own labours or by the assistance of our friends, from the two counties ; and 

 the conclusion arrived at by Mr. Lonsdale respecting the age of the South 

 Devon limestone, we now apply without reserve both to the five groups of our 

 North Devon section, and to the fossiliferous slates of Cornwall. 



This is undoubtedly the greatest change which has ever been attempted at 

 one time, in the classification of British rocks. But all the facts brought to 

 light by those, who have laboured in the same field with ourselves, were gra- 

 dually leading towards the same point ; and the establishment of the Silurian 

 organic types must before long have made the change inevitable*. 



So far from thinking ourselves rash and hasty in drawing the preceding 



* The reasons for the new classification of those stratified rocks of Devon and Cornwall which 

 are below the culm-measures, were briefly stated by the authors in a paper published in the Phi- 

 losopliical Magazine for April 1839. 



VOL. V. SECOND SERIES. 4 U 



