224 Cotr—On the Geology of Slieve Gallion, in the County of Londonderry. 
is due to intermingling with the granite. One of them, a grey flinty-looking rock, 
from the Letteran Hollow, in the neighbourhood of the felspathic andesite above 
described, proves to have a hardness of only about four, which removes it from the 
ordinary devitrified rhyolites or rhyolitic andesites. Under the microscope, its 
extremely altered character is apparent; and veins of both calcite and dolomite 
traverse it. Relics of porphyritic crystals of biotite can be recognised. The ground- 
mass of quartz and felspar, lying between decomposed crystals of plagioclase, is in 
great part minutely micropegmatitic, and in part merely microgranular. It is 
difficult to say whether this rock represents a phase of the biotite-granite that lies 
below it, or of the older andesitic series that stretches above it to the plateaux of 
Sheve Gallion. 
Holocrystalline representatives of the andesites, ¢.e. true aphanites or dolerites, 
naturally occur in association with the lavas; but none of these appear to have 
cooled at any considerable depth. With one exception, we have to go to Lough 
Fea and Lissan before we meet with anything so coarse as a diorite or a 
gabbro. 
I have noted aphanite—using the term to cover both amphibole-plagioclase 
and pyroxene-plagioclase rocks—at Tirgan Rock, in a field to the north of the more ~ 
conspicuous rocky bosses; at the south end of Drummuck; on the crest of Tintagh 
Mountain; at several points above the combe of the Derryganard Hollow ; and in 
the little cliffs between the bog of Glenarudda Mountain and the gravel hills of 
Crockandun. It also occurs, associated with true hornblende-diorite, on an inter- 
esting boss in Letteran, and in Tatnagilta, south-west of Lissan. 
A typical example of these holocrystalline rocks, closely allied to the surround- 
ing compacter andesites, is that from the north-west side of Tintagh Mountain. 
It is green-grey, speckled with yellow dots of epidote; and patches of pyrite 
emphasise its altered character. In section, it is a typical diabase, an altered 
dolerite, consisting of rod-shaped crystals of plagioclase, brown augite ophitically 
surrounding them, and abundant magnetite. Chlorite has developed along the 
cracks of the augite, and needles of green secondary amphibole spread from the 
crystals, penetrating the adjoining felspars. These needles jut out parallel to one 
another, forming delicate palisades, the components of which are often similarly 
oriented at opposite ends of the originating crystal of augite. They thus differ 
from the ordinary fibrous zones of the “ flasergabbros,” and may occur at two 
terminal regions of a crystal and not upon its other sides, suggesting a polar 
arrangement. 
In the section examined, two lumps of foreign material occur, no doubt of 
sedimentary origin, and picked up by the intrusive dolerite; the contact-zone 
formed round them is marked by the small size of the augite and felspar crystals, 
which developed rapidly against the derived fragments. Plates of biotite, stretch- 
