242 JOURNAL OF THE PLYMOUTH INSTITUTION. 
with occasionally at the junction of granites and slates. These 
latter forms of gneissic structure are, however, the casual results 
of contact-metamorphism. 
Contact-Alteration.— We pass to our examples of contact- 
metamorphism—the changes that have taken place in the normal 
condition of the rocks along the edge of the Dartmoor granite, as 
a consequence of its intrusion. Rocks, as a whole, may be 
regarded as in a state of unstable equilibrium. They are not 
only always susceptible of, but always undergoing, change, as the 
result of mechanical or chemical agencies. The zone of slates 
and grits, &c., immediately skirting Dartmoor has been greatly 
influenced by the uptrusion of the granite, and what is more, 
a reactionary influence has been produced upon that rock. The 
special presence of schorl in the outer area of our granitic 
masses, already noted, is, for example, due to a change that has 
taken place in the original character of the granite; and schorl 
rock in its different varieties is an altered form. How far we are 
to connect the occurrence of this mineral with the original granitic 
upthrust is another matter. The fact that in many cases it 
evidently replaces the primary minerals that crystallised out of 
the granitic magma, countenances the theory that it is of later 
date, and possibly due to the passage upward of chemical vapours, 
such as are sometimes presumed to have changed the felspar into 
kaolin. 
The change in the granite to which I specially refer, is, 
however, contemporary with its formation, and illustrated by 
actual contact junction specimens. The best example that I have 
seen of a contact hand-specimen from Lee Moor shows the granite 
in a semi-decayed state, the felspar with an earthy texture. 
Tourmaline has been developed along the immediate line of 
junction, and the original slate has been converted into a well- 
marked mica-schist. In no instance have I found so much 
reciprocal alteration, with such a clear preservation of the dis- 
tinguishing crystalline-granular and schistose characters. 
A contact specimen from Ivybridge shows precisely the same 
change in the granitic constituents, the most marked being the 
earthy aspect of the felspar. The slates are changed into a grey 
micaceous rock, in which the original structure is to a great extent 
obliterated. Schorl has developed in the granite. 
