218 CoLtr—On the Geology of Slieve Gallion, in the County of Londonderry. 
the ‘ pyroxenic rocks.” This point is not faced in the memoir; and we should have 
been clearer if the authors responsible for sheets 26 and 27 had declined to fit the 
northern parts of their maps together, or had definitely placed the ‘‘ pyroxenic 
rocks ” on two horizons. 
In the longitudinal section, sheet 27, published in 1889, ‘‘ metamorphic rocks ” 
and ‘‘hornblendic rocks of several varieties” occupy the position of the diabase 
series, and they are shown graduating into schists on Fir Mountain to the west, as 
no doubt they actually do.* 
J. F. Berger, in 1816, observed ‘“ several unconnected masses of greenstone”’ 
upon Tintagh Mountain, described their veins of calcite, barytes, and red iron- 
stone, and clearly distinguished them from the ‘flat stratified trap” above the 
Chalk. Personally, I think that we have good grounds for linking even the coarse 
pyroxenic rocks of Oritor with the andesitic series of Slieve Gallion, as Mr. Nolan 
actually did in 1884. But the question of their age isa large matter, which cannot 
be entered upon here. I propose first to describe some of these ‘‘ pyroxenic rocks” 
and their mode of occurrence in the field. 
In the first place, with the exception of certain chert bands, I can find nothing 
in the supposed Ordovician series of Slieve Gallion to suggest a normal sediment, 
though such may exist in sections, now concealed, in the extreme east of the area.t 
Wherever I can find the rock exposed, it seems at least as ‘‘ pyroxenic” a type as 
in the regions coloured darkly on the map; and at Tirgan Rock and Carncose 
Rock, which are included in the sedimentary area, the bosses seem to me entirely 
igneous. 
But a gradation from the compact igneous type to a bedded condition 
undoubtedly occurs ; and it is this that has probably led to a misapprehension of 
the diabase series.§ Where the townlands of Carncose and Tirgan abut upon 
the mountain, a number of rocky terraces and knobs break the ascent. On the 
lowest of these, Drummuck, there are grey flinty rocks, sometimes dark, some- 
times pale, with an almost shaly structure, and a distinct dip towards the north. 
The summit of Drummuck is formed of vesicular andesite, and the south end shows 
an aphanite, probably intrusive. In microscopic section, the bedded rock is seen 
to consist of granular plagioclastic felspars, heaped together, with some biotite and 
altered pyroxene. Epidote occurs, as it does in the whole diabase series. There 
are no obvious scoriaceous particles, such as might prove the deposit to be volcanic ; 
* See also Mem. sheet 26, pp. 12 and 15; Portlock, ‘‘ Geol. of Londonderry,” p. 544; and Mc Henry 
and Watts, ‘‘ Guide to the Rocks and Fossils, Geol. Sury. of Ireland,” p. 73. 
+ “Geol. Features of the North-eastern Counties of Ireland,” Trans. Geol. Soc. London, vol. ii., p. 149. 
t Mem. sheet 27, p. 10; Glenview, for example. 
§ Sir R. Griffith, in the edition of his Geol. Map of Ireland dated 1855, colours this series as ‘‘ porphy- 
ritic slate in which the sedimentary lines are still traceable, passing into greenstone porphyry.” 
