Srvmour—Wetamorphism of a Porphyritic Andesite. 573 
noticeable is the tendency to form larger crystalline constituents, 
especially in the case of the biotite. A section of the rock in the 
final stage shows frequent knots or bunches of this mineral in the 
ground, the individual crystals of which are of much larger size 
than usual, and considerably larger than those of the ground at 
the expense of the smaller individuals of which they have been 
developed. The occurrence of larger biotite crystals in the late 
stage of the dynamic alteration of the Donard rock was noted by 
me in 1899', but its significance was not recognised till after 
reading recently a Paper by Van Hise’ dealing with metamor- 
phism. This author notes, in discussing re-crystallization, that, as 
the process of granulation goes on, a stage is reached beyond which 
the particles do not become more finely granulated, but a 
reverse process takes place, and instead of becoming smaller they 
become larger. He regards the coarsely crystalline, perfectly schis- 
tose rocks, nearly free from strain shadows as representing the 
most advanced stages of metamorphism. 
The age of the shearing movement which has effected the 
Donard rock above described is clearly contemporaneous with that 
of the intrusion of the Leinster granite. Professor Sollas? believes 
that the granite laccolite was injected in successive sheets part passu 
with the tolding of its Ordovician cover, and that this movement 
continued for some time, and was very slow. Evidence is adduced 
for this by the presence of fractured contact minerals (garnets) 
produced in the neighbouring slates surrounding the granite’ mass. 
Confirmatory evidence is afforded by the Donard rock, for the 
shearing has been evidently superimposed on a rock already meta- 
morphosed by contact with the granite. When it was slightly 
sheared, an intrusion of granite took place, veins from which (still 
found unsheared in the Deerpark area) traversed the basic rocks. 
Other veins of aplite have, however, been highly sheared in the 
northern locality (Ballymooney) due to further movement after 
their intrusion. Veins of quartz cutting the basic rocks are also 
sheared by the same movement and in the same direction, but 
it is uncertain whether these quartz veins are of granitic or earlier 
origin. 
1<¢ Summary of Progress,’’ 1899, p. 178. 
# Bull. Geol. Soc, Am., vol. 9, 1898. 
3 Proc. Geologists’ Assoc., vol. xiii., 1893. 
