1867. J The Ljmgenous Mocks near Montbrison, 21 



Now both to the north and south of Montbrison, is descried, 

 elevated above the general level of the granitic formation, a number 

 of isolated knolls rising abruptly to a height of 500 feet or 

 upwards, and in general capped by the ruins either of a church, 

 a convent, or a castle, for which these summits would have been 

 especially well adapted, both as being conspicuous objects from a 

 distance, and also irom their steepness being secure from assault. 



We visited several of these little detached hills, as for instance, 

 St. Eomain-le-Puy to the south, and Marcilly-le-Pave and Mont- 

 verdun to the north of Montbrison, and found each of them com- 

 posed up to its summit of basalt, which also extended nearly down 

 to the level of the surrounding country. 



At St. Eomain-le-Puy, and Marcilly-le-Pave, the trap rock rested 

 upon granite, but that of Montverdun was incumbent directly upon 

 the tertiary formation, which, as before stated, is superposed upon 

 the granitic rocks on the lower levels. 



Moreover, to the east of the road leading from Montverdun to 

 Montbrison is a ridge, the longer axis of which lies nearly from 

 north to south, wholly made up of the same material. 



About half an English mile from Montbrison itself, at a place 

 called " Le Eoche," occurs the most instructive section which came 

 under our notice ; for here about halfway up the hill the basalt 

 may be distinctly seen intruding itself into, and thrust through the 

 midst of the granite, which is in consequence uplifted, and manifests 

 itself both above and below the igneous rock, in the quarry, where 

 the latter for road purposes is extensively worked. 



Indeed the granite occupies a much more elevated position than 

 this on the hills to the west of the spot where the basalt is seen, 

 for the latter is found only at a certain elevation, being bounded 

 both above and below by the granite of the country. 



Judging from these facts, which are thoroughly borne out by 

 the negative evidence, stated in the former part of this commu- 

 nication, I should conclude, that a vast antiquity must be assigned 

 to the basalts which occur about Montbrison, for one can only 

 account for the isolated position in which they are found on the 

 detached knolls scattered over the district, by supposing that they 

 constituted a part of one great continuous sheet of volcanic materials, 

 which once overspread the surface, and of which the intervening 

 portions have been since removed by denudation. 



Of course such a supposition removes their origin to an 

 immeasurable distance in point of time from any physical convul- 

 sions of recent or historical date, and indeed from the whole modern 

 class of volcanos which has been described in my former memoirs 

 on this country. They remind one of the basaltic eminences met 

 with in Saxony, which Werner referred to his imaginary floetz-trap 

 formation, with reference to which we are also compelled to assume 



