380 RELATIVE AGE OF 



[Ch. XXVI. 



but in order to see them, we must travel to lands near the 

 equator. 



In like manner, if the hypogene rocks can only originate at 

 great depths in the regions of subterranean heat, and if it 

 requires many geological epochs to raise them to the surface, 

 they must be very ancient before they make their appearance 

 in the superficial parts of the earth's crust. They may still be 

 forming in every century, and they may have been produced in 

 equal quantities during each successive geological period of equal 

 duration ; but in order to see them in a nascent state, slowly 

 consolidating from a state of fusion, or semi-fusion, we must 

 descend into the ( fuelled entrails ' of the earth, into the regions 

 described by the poets., where for ages the land has 



ever burn'd 



With solid; as the lake with liquid fire. 



As the progress of decay and reproduction by aqueous agency 

 is incessant on the surface of the continents, and in the bed of 

 the ocean, while the hypogene rocks are generated below, or 

 are rising gradually from the volcanic foci, thus there must 

 ever be a remodelling of the earth's surface in the time 

 intermediate between the origin of each set of plutonic and 

 metamorphic rocks, and the protrusion of the same into the 

 atmosphere or the ocean. Suppose the principal source of the 

 Etncan lavas to lie at the depth of ten miles, we may easily 

 conceive that before they can be uplifted to the day several 

 distinct scries of earthquakes must occur, and between each of 

 these there might usually be one or more periods of tranquillity. 

 The time required for so great a development of subterranean 

 t-levatory movements might well be protracted until the depo- 

 sition of a series of sedimentary rocks, equal in extent to all 

 our secondary and tertiary formations, had taken place. We 

 conceive, therefore, that the relative age of the visible plutonic 

 and metamorphic rocks, as compared to the unaltered sedimen- 

 tary strata, must always IK? determined by the relations of t\vo 

 forces, the power which uplifts the hypogene rocks, and that 

 aqueous agency which degrades and renovates the earth's 



