Cotr—On the Geology of Slieve Gallion, in the County of Londonderry. 227 
the rock together. A quartz-diorite thus results, from the union of a much-decom- 
posed gabbro with a fresh and active magma, richly endowed with silica. 
The specific gravity of this rock is 2°76. A diorite from the south of Mobuy, 
which may, I think, fairly represent the original condition of the Letteran boss, has 
the far higher specific gravity of 2:95; while the diallage-gabbro midway between 
Lissan and Oritor, which is almost certainly another member of the same series, 
gives 2:98. On the other hand, the grey eurite on the shoulder west of the quartz- 
diorite of Letteran has a specific gravity of 2°66. These figures are certainly 
suggestive of the amount of new material which was received by the diorite of 
Letteran from the invading highly siliceous magma. 
Although, as we have said, the occurrence of pink felspars in a rock of the dia- 
base series is not sufficient to prove transference of material from the granite, yet 
it is easy to see in the field, on the west side of the Letteran boss, a ‘‘ passage” from 
the one type of rock into the other.* Eurites, with pink crystals of true ortho- 
clase, occur here, graduating naturally into granite ; and the quartz-diorite provides 
a genuine passage-rock between these and the basic series of the district. 
The microscopic characters of the quartz-diorite, and the deductions that seem 
to follow from them, agree with those given by Prof. Sollastin his remarkable 
paper on ‘the Relation of the Granite to the Gabbro of Barnavave, Carlingford.” 
Uralitic hornblende replaces the pyroxene of the Carlingford gabbro at its contact 
with granite veins: but, when occurring as derived crystals in the granite, the 
“ diallage directly passes into a sage green hornblende.” In other cases, biotite 
has arisen as the paramorphic product. The derived crystals of bytownite, which 
is the prevailing plagioclastic felspar in the Carlingford rock, have been injected 
with orthoclase, or with orthoclase and quartz, and, although dusty and corroded, 
have been restored in some cases with an external zone of orthoclase. 
Similar zonal additions to derived crystals of plagioclase seem to have occurred 
at the junction of granite and gabbro on Carrock Fell.{ Mr. Harker§ has also 
shown that hornblende, presenting good crystal-outlines, has ‘crystallized out 
from the modified granophyre-magma” at a similar junction in Skye, and also on 
Carrock Fell and in St. Kilda.|| These facts, then, still further support the 
explanation given above of the idiomorphic character of the Letteran hornblende 
when in contact with the intrusive magma. 
Moreover, a boulder from the west slope of Glenarudda Mountain gives evidence 
* See Mem. sheet 27, p. 12. 
+ Trans. R. Irish Acad., vol. xxx. (1894), pp. 494 and 495. Also Wature, vol. xlviii. (1893), p. 109. 
+ A. Harker, ‘‘ The Carrock Fell Granophyre,” Quart. Journ. Geol. Soc. Lond., vol. li. (1895), p. 137. 
§ ‘On certain Granophyres, modified by the incorporation of Gabbro-fragments, in Strath (Skye),” 
ibid., vol. lii. (1896), pp. 824 and 325. 
|| Note in paper by Sir A. Geikie, ‘‘ Tertiary Basalt-plateaux of North-western Europe,” 7id., vol. lii., 
p. 393. 
