THE ROCKS OF PLYMOUTH. 245 
micaceous-schist of thoroughly distinctive character, fine or coarse 
as the case may be. Occasionally the process is carried still 
further to the production, as already noted, of patches of pseudo- 
gneiss, with the foliations of the mica, felspar, and quartz fairly 
marked. It may be taken as a general rule that the appearance 
of mica in these slates is a proof of contiguity to the granite, and 
that the quantity of mica increases steadily up to the point of 
junction. 
There is considerably more variety in the character of the 
actual altered contact rocks, however, than might be imagined. 
We meet at times with small veins of granite passing into and 
through the adjacent slates without effecting any noteworthy 
change, the body of the injected material being apparently too 
small for that result. There seems indeed in these cases to be 
more generally a reaction upon the granitic matter, shown in the 
coarseness of its crystallization, I have also observed that in 
many veins mica is almost or wholly absent. Veins in some cases 
traverse the tourmaline-schist. 
As a rule a very distinct change is effected where the granite in 
bulk comes into contact with the rocks through which it was: 
thrust, and this commonly shows itself not only in the coarseness 
of structure of the granite, but in an exceptional development of 
felspathic constituents. This too is very well seen at Shaugh. 
Some of the altered rock touching the granite is exceedingly 
compact, and has lost all trace, or all distinctive trace, of the 
fissile structure which it originally possessed. Whether this 
obliteration is due mainly to pressure is doubtful ; but there is one 
contact rock of a very remarkable character, which undoubtedly 
has been in a fused or semifused condition, and in which the 
marks of secondary foliation are not wholly removed. It is now 
a pinkish felstone, with micro-porphyritic quartz, and grains and 
radiating prisms of tourmaline, answering to the rock which has 
received from the continental petrologists the name of hornfels. 
I have only found it in association with the granite at Shaugh. 
One of the most interesting examples of altered slate is found 
at Meavy, on Ringmoor. It has a talcose aspect, and the 
andalusite is developed in little lenticular spots of a pale bluish- 
green, as admirably shown in a section across the lamine. This 
seems to occur at a considerable distance, relatively speaking, 
from the granite. The spots of andalusite are very regular in 
VOL. IX. S 
