200 NATURAL SCIENCE. March, 1894. 



conveniently term the " upper saturation limit " of igneous material. 

 There is no reason to believe that a similar saturation is not 

 going on now under volcanic areas. Nevertheless, the vast depth 

 required for the rocks to have a sufficiently high temperature for 

 the extremely slow cooling of the giant granites, points to an equally 

 vast time to allow for denudation to expose them at the surface. 

 This at once suggests the great antiquity of areas in which the 

 phenomena referred to are seen. It is, however, well within 

 possibility that, in special areas where denudation has acted 

 powerfully for a long period, this phase of intrusion may be exposed, 

 though it took place in post-Cambrian times ; but the proofs of 

 it should be placed beyond reasonable doubt. 



George Barrow. 



