CoLtE—On the Geology of Slieve Gallion, in the County of Londonderry. 225 
ing inwards from the surface, have arisen in the fragments themselves. The best 
preserved inclusion is clearly formed of granular calcite ; and the conversion of 
this particle of ancient limestone into a biotite-calciphyre reminds one in miniature 
of the well-known blocks of Monte Somma. 
The felspar crystals of this dolerite are frequently bent by earth-pressures ; 
and other doleritic rocks occur on Slieve Gallion in which a brecciated structure 
has been produced, almost obliterating their original well crystallised character. 
The first mineral to disappear in such cases is the pyroxene ; and green streaks 
and patches of chlorite fill the interspaces between partially rounded felspars. It 
is possible that actual fusion has begun in such instances, and that the invasion of 
the granite is responsible for much that looks like brecciation, The augite would 
form a fluid ground in which the undestroyed crystals of lime-felspar and magnetite 
would be borne along* ; the mass would thus yield very readily to deformation ; 
and, on its final consolidation, disturbed and rounded felspars would be found lying 
in a basic glass. The alteration of this glass would give the green interstitial 
patches that we see to-day. A rock to which the foregoing description applies 
occurs north-west of Glenarudda Mountain, and is at the most fifty feet above the 
surface of the granite. The diabases immediately to the south-west actually le 
at a lower level than the granite bosses of Glenarudda, and are simply seamed 
with eurite viens. 
Quartz occurs in these altered dolerites, but in trifling amount, and clearly as 
a secondary product. We have now to consider a more difficult case, that of the 
aphanite of the Letteran Hollow, in which quartz is an important constituent of 
the groundmass. 
From the flinty character of the contact-zone, from the remarkable degree of 
penetration of the diabase series by veins of eurite, and especially from the 
abundant granite veins in the coarse diorites to the west, we are led in the field to 
expect an actual intermingling of the two types of material, the one of highly 
siliceous, and the other of basic composition. Portlockt clearly observed some- 
thing of this kind, although he regarded the granitic veins as produced by the 
‘* progressive action” of metamorphism in the ‘ hornblendic rocks,” and noted 
red felspars as developed by similar metamorphism in these masses. Mr. Egant 
also speaks of a passage from the one rock-type to the other, and is supported by 
Mr. Nolan, both in his earlier and later’ memoirs. 
* On this selective melting of the constituents, compare Biickstrém, ‘‘ Uber fremde Gesteinseinschliisse in 
einigen skandinavischen Diabasen,”’ Bihang till kongl. svenska Vetenskaps-Akademiens Handl., Bt. 16, Afd. 
ii., p. 10; G. Cole, ‘On derived crystals, &ec.,” Trans. Roy. Dubl. Soc., vol. v. (1894), p. 246; and A. 
Harker, ‘‘ The Carrock Fell Granophyre,” Quart. Journ, Geol. Soc. Lond., vol. li. (1895), p. 136. 
+ ‘‘Geol. of Londonderry,” p. 556 ; also p. 535. 
t Mem. sheet 27, pp. 10, 12, and 15. 
§ “Metamorphic and intrusive Rocks of Tyrone,’ Geol. Mag., 1879, p. 159, and Mem. sheet 26, 
pp. 10 and 11, 
