2+2 Pro^Sedgwlck a7id Mr. Murchison 072 the Classification 



AtlienfpufA,) in which we pointed out an ascending series of 

 ancient stratified rocks, the greater part then supposed to be- 

 long to the upper Cambrian, and a comparatively thin band to 

 the lower Silurian strata : and we further showed that the culm 

 measures of Devon, so far from being mere courses subordinate 

 to these older rocks, constituted, in fact, a vast basin or trough 

 of carbonaceous deposits, differing from the rocks on which 

 they rested, in mineral character and in organic remains. We 

 also exhibited a map in which the boundaries of this great car- 

 bonaceous trough were defined, with as much accuracy as the 

 short period of our examination permitted ; and we stated our 

 belief, that at least all the upper part of it was the equivalent 

 of the true coal measures. We further proved, that the granite 

 of Dartmoor had been protruded after the consolidation of the 

 culm or coal formation. Subsequently, we embodied these re- 

 sults with other details, not presented to the British Asso- 

 ciation, in a second memoir, read before the Geological Society 

 (June, 1837), in which we gave a short account of the structure 

 of South Devon ; the different rocks of which we placed in 

 parallel with their equivalents in North Devon, referring them, 

 as well as we were able, to their types in other districts of Great 

 Britain. 



Adhering strictly to our first view, viz. that the great over- 

 lying culm-bearing trough is the equivalent of the carbonife- 

 rous system; we proceed to point out the reasons which 

 induce us to make a material change in the classification and 

 equivalents of the older rocks of Devon and Cornwall. 



North Devon. — In our previous communications we subdi- 

 vided the part of Devon which lies between the north coast 

 and the trough of culm deposits into five ascending mineral 

 masses, closely linked to, or passing into one another. From 

 its lithological character, from the ferruginous impressions of 

 stems of Encrinites, and the resemblance of certain casts of 

 shells (much distorted, however, by compression, and by lines 

 of slaty cleavage) to fossils of the Caradoc sandstone, we 

 placed the fifth group in the lower part of the Silurian system; 

 consequently the four inferior groups (though different in many 

 respects from anything we had ever seen among the older 

 rocks of Wales or Cumberland) necessarily fell into the upper 

 portion of the Cambrian system. Over all these groups came 

 the culm measures, and certainly without the intervention of 

 conglomerates or any manifest discordancy of position. 



Since our memoirs were read, Mr. Weaver examined the 

 neighbourhood of Barnstaple ; and confirming our views as to 

 the age of the culm-measures, reported them to be uncon- 

 formable to the older rocks*. One of the authors, for the ex- 

 * Procccclincs of the Gcol. Soc. vol. ii. — . 



