USE OF THE TERM " METAMORPHIC." 79 



practice, may arise from adopting an opposite course, while 

 no inconvenience can result from the use of the term " me- 

 tamorphic," which merely implies that the rocks under 

 review have suffered intense mineral change, but advances 

 no opinion as to their age or chronological co-ordination. 

 We might colour on our geological maps the schists of the 

 Scottish Highlands and Scandinavia, as Silurian, Cambrian, 

 or Laurentian, and support our views by many plausible 

 arguments, but nothing would be gained by such a course 

 which is not already secured by the term " metamorphic," 

 while the subsequent discovery of their real character would 

 only be embarrassed by these hypothetical distinctions. 

 Let us continue, then, to treat these old rocks simply as 

 " the metamorphic," labouring to reveal their true nature 

 by the discovery of unobliterated fossils, and encouraged by 

 the success which, within the last thirty years, has resolved 

 so much of them into Silurian, Cambrian, and Laurentian 

 life- systems. 



Such is the nature of metamorphism, or that internal 

 mineral transformation to which all rock -matter in the 

 earth's crust is incessantly subjected. Pressure, heat, che- 

 mical action, and the other agents above alluded to, are con- 

 tinually solidifying, hardening, and crystallising; and no 

 sooner is a sediment laid down, or an igneous mass ejected, 

 than it begins to be operated upon by one or other of these 

 forces. Of course, the latest laid down will have suffered 

 less change than the older and deeper-seated ; hence it is 

 chiefly among the latter that metamorphism is to be seen in 

 its greatest intensity. It may happen in certain areas, such 

 as centres of vulcanic activity, that secondary strata may be 

 as much metamorphosed, or even more so, than any of the 

 primary ; but still such instances are exceptional, and it 

 may be safely asserted as a general rule that the older 



