4- PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May l^, 



anH association as at Littry, being independent of the older strata, 

 and connected with the new red sandstone. M. de Gerville states 

 that the Plessis mine has afforded him about a dozen species of Ferns 

 and Calamites, and these I had an opportunity of seeing in that gen- 

 tleman's rich collection. The only plant quoted by M. A. Brongniart 

 from the mines of Plessis is Neuropteris rotundifoliay and this has 

 not hitherto been recognised in any other locality. 



The circumstance which first suggested that the coal-beds of this 

 part of France were distinct from the true coal-measures, was rather 

 the relation which a peculiar porphyritic rock bears to the several 

 deposits, than any considerations as to position, on which point,a 

 mere traveller through a district has hardly ever time to bestow suf- 

 ficient investigation. 



Every geologist of the present day is ready to admit that the great 

 erupted masses of each distinct period preserve, over areas of con- 

 siderable geographical extent, not only an identity in composition, 

 but also certain obvious external characters by which they may be 

 recognised at once, such as the porphyritic granites, for instance, or 

 what is more particularly in point, the red quartziferous porphyry of 

 Exeter, (and the porphyres quartziferes rouges) of Brittany and the 

 Cotentin. Throughout the latter districts of France the eruption of 

 these masses has taken place at some period subsequent to the con- 

 solidation of the paleeozoic strata ; so much so, that I believe MM. 

 Dufrenoy and Elie de Beaumont connect this outbreak with the 

 disturbance which closed that great period. This view is indeed 

 founded on facts gathered in many parts of France, but the proofs of 

 it are more direct and evident in the west of England than I found 

 them elsewhere. An account of these porphyries may be seen in Sir 

 H. De la Beche's " Report on Devon and Cornwall*," and in a me- 

 moir by myself already reported in the Geological Proceedings f. 

 This red quartziferous porphyry occurs in association with the lower 

 beds of the several coal-fields of Calvados and La Manche. M. He- 

 rault has particularly noticed it with respect to the Littry mines, and 

 he also infers its greater antiquity from the fact that it has supplied the 

 shingle which enters so largely into the beds of conglomerate which 

 underlie the coal : at Littry the strata are horizontal, but at Plessis, 

 where they are much broken, the disturbances have, I believe, been 

 considered by some geologists to have arisen from the intrusion of 

 the porphyry, just as we still find it stated occasionally that the erup- 

 tion of the Exeter porphyries was the cause of the Devonshire con- 

 glomerates. At Plessis, as at the other places, the mechanical strata 

 contain portions of the erupted rock, and consequently are subsequent 

 to them. Now the precise period of the appearance of the red porphy- 

 ries was at the close of the carboniferous series. Their mode of associ- 

 ation with the lower beds of the Littry deposits is similar to that which 

 has long been familiar to English geologists with respect to the west 

 of England conglomerate, and thus these Littry beds must be referred 

 without any doubt to the new red sandstone period. It is, however, 



* Report, &c., p. 204. f Geol. Proc, vol. ii. p. 587. 



