Cote—On the Geology of Slieve Gallion, in the County of Londonderry. 217 
fresh information as to the petrography of the area; but the basic series of Slieve 
Gallion itself remains practically unnoticed. Despite previous investigations, the 
mountain is sufficiently remote from the routes of ordinary travellers to offer an 
attractive field; and I hope that my own observations may at least indicate its 
interest. 
The occurrence of ‘‘andesitic volcanic tuft” among the pyroxenic rocks of 
Slieve Gallion, and of highly silicated and basic igneous rocks at Oritor, were 
recorded by myself in the reports of the Dublin Microscopical Club during 
1896.* I now propose to describe the various masses referred to, in their field- 
relations and in their bearing one upon another. 
Il.—Tuere ANpDEsIVeE AND Diortre SERIES. 
The series thus designated includes the whole of the ‘‘ Pyroxenic Rocks” dealt 
with in the Memoir to sheet 27 of the Geological Survey. They are mostly 
chloritic, forming, in fact, typical diabases,} using that term in its wide and practical 
significance ; but their resemblance to the similar grey-green altered rocks of other 
areas leaves no doubt as to theirigneous origin. Respect for the views of Portlock, 
who regarded them as metamorphic products in 1843, may have prompted the 
somewhat remarkable statements of the Survey Memoir,{ published in 1881; but 
the work that had been done meanwhile in other areas of the British Isles, not to 
mention the investigation of numerous diabases and schalsteins on the Continent, 
had already shown how igneous rocks may come to resemble sediments or schists, 
and how certain characters may none the less remain, by which their true mode 
of origin may be traced. 
In the system of colouring adopted in sheet 27, we are asked to regard these 
“« pyroxenic rocks” as graduating into Ordovician sediments, and as produced by 
metamorphism from them. In sheet 26, which includes the west end of Sleve 
Gallion, they are represented in precisely the same relation to Ordovician beds 
(“B1to2”); but in the index to that sheet, as in the accompanying memoir,§ 
they are said to be ‘ probably of pre-Cambrian or Upper Laurentian age.” They 
are thus swept into the series that forms the moorland west of Cookstown ; but 
part, at any rate, of the supposed Ordovician strata of this sheet becomes drawn 
back also, by the same stroke, into the Archzean era, in virtue of its relations to 
* Trish Naturalist, vol. v., pp. 245 and 312. The remarks in the second reference apply to the neigh- 
bourhood of Oritor better than to Oritor Quarry. 
+ J. F. L. Hausmann, “ Ueber die Bildung des Harzgebirges ” (1842). Zirkel, ‘‘ Lehrbuch der Petro- 
graphie,” 2nd edit., Bd. ii. (1894), p. 650. 
{ Mem. sheet 27, p. 11. 
§ Mem. sheet 26, pp. 12, 15, and 16. 
