Ch. XL] THE PRIMARY ROCKS. 215 



might be brought into contact. The theory requires these 

 periodical metamorphoses, and its supporters assert that 

 nature exhibits such primary strata, not only associated with 

 the greywacke, carboniferous, oolitic, and cretaceous groups, 

 but actually passing into these stratified rocks, which were 

 formerly supposed to be of more recent origins. 



Granite is, therefore, now considered as a rock of all ages ; 

 inasmuch as the central igneous mass may be at any period 

 injected among the superincumbent deposits : it remains to 

 be stated how its precise age is to be ascertained. This point 

 cannot be so easily determined as might be conceived on the 

 first propounding of this subject; for granite in the vicinity 

 of chalk, may be more ancient than that in connection with 

 the oldest fossiliferous strata ; because, in the latter case, the 

 catastrophe by which the granite was raised, in a state of fusion, 

 might not have been adequate to the propulsion of the mass 

 beyond the limits of these deposits. How, then, is this point 

 to be ascertained ? The relative position of the strata is sup- 

 posed to answer this indication : for, their inclination being 

 attributed to the elevation of the granitic mass, by which the 

 strata were broken through and tilted up at various angles, 

 it is presumed to follow, that, if we find other stratified rocks 

 resting thereon in a horizontal position, the period of the 

 granitic protrusion must have been in the interval between 

 the formation of the most recent of the inclined strata, and 

 the deposition of the incumbent rocks. This interval, how- 

 ever, is frequently one of very considerable latitude, em- 

 bracing several geological epochs : it is, therefore, only when 

 the superimposed rock is the immediate successor of the 

 newest upraised stratum, in a regularly consecutive series, that 

 the age of the granite is most distinctly indicated ; and even 

 then, it can only afford an approximation to the truth, inas- 

 much as the precise lapse of time between these events has 

 not been ascertained. 



On this principle, M. Elie de Beaumont has recently en- 

 deavoured to determine the relative ages of the various moun- 



p 4 



