Geology and Physical Geography. 309 



ably very well have passed, if lithological structure and 

 aspect alone determined their age. The schists, porphyritic 

 grits, and other varieties of the sedimentary poHions of these 

 rocks, as well as the various porphyries by which they are 

 penetrated, are indeed well depicted by M. Dufrenoy, in the 

 Memoires pour Servir ; and the very locality in question was 

 specially described by M. Visquesnel. These authors never 

 observed fossils ; and hence, judging from mineral character 

 only, they have assigned much too liigh an antiquity to the 

 rocks. Sir Roderick having ascended the Sichon from the 

 Castle of Busset, for some leagues, to near its source (Fer- 

 rieres), found tliat the eruptions of the porphyry (often very 

 granitiferous) and the schistose rocky grits were unusually so 

 metamorphosed and dislocated, that he was wholly unable 

 to attempt to define anything like a descending series from 

 the above-mentioned lower carboniferous rocks to others, 

 which might be considered Devonian and Silurian. Besides 

 fossils, he observed two thin bands of scaly, hard, subcrystal- 

 line limestone associated with the schists of Busset. In truth, 

 the limestones increase in volume near Ferrieres, where they 

 are in the state of marble, of grayish, reddish-veined, and even 

 of white colours. In order to satisfy himself whether the 

 loftier and bolder portions of the chain of the Forez be- 

 longed to the same class of rocks as those he had ex- 

 amined in its lower and northern end (though they are 

 distinguished in the map of France), Sir Roderick made an 

 excursion to Thiers. There he recognised precisely the 

 same phenomena, but on a grander scale, and in broken and 

 picturesque gorges, as he had witnessed along the banks 

 of the Sichon and around the Castle of Busset. Towering 

 masses of dark gray and reddish quartziferous porphyry, 

 with veinstones of quartz (occasionally very like granite), 

 have there penetrated in every direction the mutilated schists 

 and grits, which can, notwithstanding, be recognised as frag- 

 ments of the very same strata as those of the Sichon ; 

 although the greywacke, like schist, has in many parts be- 

 come a sort of crystalline amphibolic schist, and the grit has 

 been converted into quartz rock. In examining the granitic 

 and schistose chain of the west of the Limagne, Sir Roderick 



