Ch, XXVI.] HYPOGENE ROCKS. 379 



which it penetrated ; but there would rarely be any date to 

 show that it might not have been injected at the close of the 

 Liassic period, or at some much later era. 



The metamorphic rocks must be the oldest, that is to say, 

 they must lie at the bottom of each series of superimposed 

 strata, because the influence of the volcanic heat proceeds from 

 below upwards ; but the hypogene strata of one country may 

 be, and frequently are, of a very different age from those of 

 another. The greater part, however, of the visible hypogene 

 rocks are, we believe, more ancient than the carboniferous for- 

 mations. In the latter, we frequently discover pebbles of hypo- 

 gene rocks, namely, granite, gneiss, mica- schist, and clay -slate ; 

 and the carboniferous rocks often rest unchanged upon the 

 hypogene. According to our views of the operations of earth- 

 quakes, we ought not to expect plutonic and metamorphic rocks 

 of the more modern eras to have reached the surface generally, 

 for we must imagine many geological periods to elapse before a 

 mass which has put on its particular form far below the level of 

 the sea, can have been upraised and laid open to view above 

 that level. Beds containing marine shells sometimes appear at 

 the height of two or three miles in the principal mountain-chains, 

 but they always belong to formations of considerable antiquity ; 

 still more should we be prepared to find the hypogene rocks 

 now in sight to be of high relative antiquity, since, in order to 

 be brought up to view, they must* probably have risen from a 

 position far inferior to the bottom of the ocean. 



We shall endeavour to elucidate the cause of the great age 

 of the plutonic and metamorphic rocks, now in sight, by a 

 familiar illustration. Suppose two months to be the usual 

 time required for passing from some tropical country to our 

 island, and that an annual importation takes place of a certain 

 tropical species of insect, the ordinary term of whose life is two 

 months, and which can only be reared in the climate of that 

 equatorial country. It is evident that no living individuals 

 could ever be seen in England except in extreme old age. The 

 young may come annually into the world in great numbers, 



