of the Older Stratified Rocks of Devonshire and Conmall. 257 



both sides of a great trough, and included the hitherto anoma- 

 lous Hmestones of Holcombe Rogus in the same carboni- 

 ferous system. 



4. That we ascertained the subdivisions of the cuhn series, 

 and laid down, for the first time, on a map, its extent (with one 

 limited and perhaps doubtful exception at its south-eastern 

 extremity near Ashburton), thus converting the supposed an- 

 cie7it greywacke rocks of nearly one-half of the large county 

 of Devon into equivalents of the carboniferous system, as it is 

 already represented in two published geological maps*. 



In attempting to classify the oldest rocks of Devon we fell 

 into some false conclusions from imperfect data. These we 

 have corrected, and the whole series of Devonshire and Corn- 

 wall is now, we trust, exhibited in harmony; the lower sand- 

 stones and slates being the equivalents of the old red sand- 

 stone, the next natural group beneath the great coal-bearing 

 strata of the British Isles, and the whole being exhibited 

 under peculiar mineral types. 



In asserting that the older stratified rocks of Devonshire 

 and Cornwall are upon a broad scale the equivalents of the 

 carhoniferous and old red systems, we do not however deny 

 that in some tracts the lowest members of these rocks may 

 represent the upper division of the Silurian System : for al- 

 though we have as yet found few if any of the fossils most ty- 

 pical of that system,we admit that when the sediments of a given 

 epoch have been accumulated under peculiar conditions, we 

 must expect to find considerable variations in the forms of 

 animal life. Again, we know that the older rocks of this 

 region have undergone great changes in assuming their pre- 

 sent hard and slaty character ; and under such circumstances 

 the difficulty of precisely limiting the boundary line of any 

 given portion of them is prodigiously increased. In tracing, for 

 example, the Silurian System from its typical region into 

 the sea-clifFs of Pembrokeshire (where its place in the series 

 is so precisely marked in Broad Sound) we perceive its ordi- 



* See small map of England, in the corner of the large map of the Si- 

 lurian region. See also Phillips's General Geological Map of the British 

 Isles. In the latter it has been inaccurately stated (in acknowledging the 

 sources from which the author drew) that Devonshire is coloured from Mr. 

 De la Beche. Professor Phillips has promised to correct this error in the 

 new edition of his map which is about to appear, as he is fully aware, to- 

 gether with every geologist who was present at the meeting of the British 

 Association, that we first proposed this great change : and even now, 

 though Mr. De la Bcche has followed us in se|)arating the carbonaceous 

 rocks from the grcywacke, and represents them under a distinct colour, 

 he does not admit them to be the equivalents of the carboniferous 

 system of England. 



Phil. Ma^. S. -3. Vol. li. No. 89. Ajml 1839. S 



