﻿1850.] 



MURCHISON — SLATY ROCKS OF THE SICHON. 



17 



they constitute an upper member of the same group. At all events, 

 the highly dislocated positions of the former, and the manner in 

 which they are wedged in among more ancient rocks, indicate that 

 the great porphyry eruptions, to which allusion has been made, took 

 place subsequently to their accumulation. 



In the meantime, the discovery of the above-mentioned organic re- 

 mains in the schists of the Sichon has enabled us to assign to one of 

 the great flanking ridges of the Limague d'Auvergne, a much more 

 recent age than that to which its rocks had previously been referred. 

 We now see that the fundamental strata of the Forez are, in fact, of 

 precisely the same epoch as those of the adjoining and parallel chain 

 of the Tarrare, which is interposed between the region now under 

 consideration and the coal-fields of St. Etienne and Lyons. The 

 French geologist, M. Griiner, had discovered in adjacent parallel 

 tracts, and notably at liegny, two leagues east of Roanne*, a cer- 

 tain number of fossils which M. Voltz and himself had classed as 

 Silurian, but which M. de Verneuil has assigned to the carboniferous 

 system f. Just as on the Sichon, these fossils occur in schists more 

 or less metamorphosed, and occasionally very crystalline, which, like 

 those of the Sichon, are also much penetrated by porphyry. 



The occurrence of mountain limestone fossils in strata of such an 

 antique and crystalline aspect, and which, being in the highest degree 

 dislocated and inclined, are stated by the French geologists to be un- 

 conformable to certain overlying coal-fields of France, determines a 

 question of deep interest to the physical geologist and palaeontologist. 

 Assuming that this unconformity exists, it follows that there must 

 have been a powerful disruption of the older members of the carbo- 

 niferous series (i. e. of strata of the age of the mountain limestone 

 with its shales and sandstones) before the overlying coal-bearing de- 

 posits were accumulated. 



The proofs of a conformable collocation in Franconia of Silurian, 

 Devonian, and lower carboniferous strata, as formerly adduced by 

 Professor Sedgwick and myself {, were doubted by an eminent French 

 geologist §, simply because the adjacent coal-fields of Bohemia had 

 not partaken of the same movements of upheaval. Hence it was sur- 

 mised that the limestones of Hof, though containing numerous large 

 Producti identical with species well known in Britain (and which in- 

 duced my friend and self unhesitatingly to consider this rock true 

 carboniferous limestone), could not really be carboniferous, but must 

 belong to an earlier sera. Again, the very decisive testimony obtained 

 by M. de Verneuil from the limestones of Sable and its environs was 

 doubted, in spite of a profusion of true carboniferous types, by those 

 who, classifying rocks by signs of physical dislocation, could not then 

 be induced to believe that so great a movement could have occurred 



* See Annales des Mines, vol. xix. p. 80, 3rd ser. 



f M. Jourdan has proved by his fine collection of fossils, that these carbonife- 

 rous strata extend from the Saone to the Loire and Allier. See Proces verbaux 

 de la Societe d' Agriculture de Lyon, vol. i. 2 serie, p. 67. 



% See Trans. Geol. Soc. Lond. New Series, vol. vi. part 2. p. 298, pi. 23. fig. 15. 



§ M. Elie de Beaumont. 



VOL. VII. PART I. C 



