38 Messrs Lyell and Murcliison un the 



pened in 1733, probably occasioned by the undermining of the 

 light puraiceous beds, which caused the fall of the more massive 

 overlying trachytic breccia. The ruins of the latter form a ta- 

 lus a mile and a half broad at its lower extremity, and many of 

 the fallen masses are of colossal size. 



There is a great fissure in the basalt, parallel to the face of 

 the hill, perhaps occasioned by failure of support on the removal 

 of the trachytic breccia, which must formerly have acted as a 

 buttress in propping up the precipitous basalt. Where the re- 

 mains of this landfall do not conceal the section, the tertiary 

 marls may be traced from the valley upwards, to within a few 

 feet of the overlying basalt. 



Before attempting any explanation of the above phenomena, 

 we shall mention two other localities of deposites analogous in 

 their component parts, and geological position, to those of Mont 

 Perrier. One of these is seen on the summit of a precipitous 

 cliff overhanging the Allier at St Maurice, near St Romain. 

 This cliff consists of tertiary grits and marls, from four hun- 

 dred to five hundred feet in thickness, still undermined by the 

 river. Upon these strata rests alluvium and trachytic breccia, 

 occupying a ledge, as it were, on the side of the hill ; higher 

 than which is seen a steep slope, consisting of gypseous marls, 

 capped with basalt. The bottom bed of the alluvium is filled 

 with rounded pebbles of quartz, granite, and porphyrltic trachyte, 

 with numerous fragments of fres^h-water marls. The pebbles are 

 stained by iron of a deep purple colour. This is about seven 

 feet thick, and supports the trachytic breccia, which is a much 

 greater mass, and is identical with that of Mont Dor. 



The other locality of trachytic breccia, mentioned by Mr 

 Scrope, is on the southern side of the hill of Monton, at the dis- 

 tance of about ten miles north of Mont Perrier. It is almost a 

 mountain mass, very pumiceous, and contains fragments of ba- 

 salt as well as trachyte, but there are no interposed beds of 

 rounded alluvial pebbles. In the tufaceous portion of this rock, 

 as at Perrier, the inhabitants have excavated subterranean 

 dwellings. Below the breccia, tertiary strata are exposed to 

 the depth of many hundred feet. They consist chiefly of thin- 

 ly laminated greoi marls, filled with cypris, and some white 



