578 MISS M. I. GARDINEK ON CONTACT-ALTERATION 



The small garnets are sometimes of a brownish red, as in the 

 altered grits, and sometimes a bright wine-red colour. Large yellow 

 garnets occur occasionally. 



Tourmaline occurs in the Knocknairling-Hill vein in patches 

 curiously intergrown with quartz, so as to form what looks like a 

 graphic granite, with the little hooked letters in dark tourmaline 

 instead of in quartz (PI. XXIII. fig. 3). The tourmaline needles are 

 parallel with each other, instead of radiating as in luxullianite. 

 Some of it is of an indigo, and some a bottle-glass brownish-green 

 colour. These tourmaline-quartz patches have an area of 1 or 2 

 square inches. Groups of tourmaline sections extinguish together, 

 but the quartz-grains have their axes at all angles. 



The fine-grained variety of the aplite has a sparkling saccharoidal 

 look, and the little pink garnets show conspicuously on the white 

 surface of a freshl3-broken specimen. It consists of about equal 

 quantities of felspar and quartz in grains. There is a good deal of 

 white mica, garnets are common, and small needles of tourmaline 

 occur occasionally. In some places coarser veins shade off in parts 

 into this fine-grained material ; and in others it occurs in veins by 

 itself. 



§ 7. Age oe the Metamorphosed Rocks. 



The evidence that the metamorphosed rocks of Knocknairling Hill 

 are of Silurian age is, I think, fairly conclusive. 



There can be no question as to the age of the grits and shales of 

 the stream. They are just like grits and shales which occur at 

 various parts of the margin. The amount of mica in the grit varies, 

 and in some parts the rock differs but little from the unaltered grits 

 of the district. It is certainly a pity that the schistose beds of the 

 stream, which I have called altered flags, cannot be traced farther 

 from the granite ; but the way in which they begin by alternating 

 with unaltered shales renders it unlikely that they have been faulted 

 in. This alternation is just like that of the flags and shales of the 

 Ardwells. A similar case of the alternation of thin, highly altered 

 bands with unaltered shales occurs at a greater distance from the 

 granite in the shales marked A on the map (fig. 1). In these there 

 are quartzose bands, an inch or two thick, with much white mica 

 and garnets. 



The grits and shales are Silurian. The mode of occurrence of 

 the schistose beds in the stream renders it probable that they are 

 of the same age. 



The more altered rocks of the hill-side seem clearly to be the 

 same series. There are unaltered shales on one side, and altered 

 flags, like those in the stream, on the other. The strike of the 

 whole, as given not only by the foliation, but by the shales inter- 

 bedded with the altered flags, is the normal strike of the Silurians 

 of the district. 



There is also evidence of a different nature. Xowhcre else is the 

 metamorphism of the rocks in contact with this granite-mass so 

 striking : but to the south, at the Clints-of-Dromore, the grits, at the 



