Trappecin Rocks, 
27 
and hydrothermal action, and of depths below the surface and 
pressure of superincumbent rocks at which these agencies were 
in operation. 
The greater number of varieties of mineral composition and 
arrangement, hardness, ‘general structure, texture, &c noticeable 
in the trappean or Newer Plutonic rocks, indicates that the con- 
ditions under which these combinations were effected, and the 
cooling of the rock-masses took place, were more varied than in 
the case of the Older Plutonic granites. 
Re-fusion may also in many cases have caused an altered 
arrangement of component minerals. 
Leaving the remaining groups of igneous rocks for consideration 
in connexion with the sedimentary deposits of the geological 
epochs to which they belong, the Lower Pakeozoic and meta- 
morphic (azoic) sedimentary rocks will now be noticed. 
