94 MK. CtAedinee and prof, eet^^olds ox the [Feb. 19 12, 



alumina being combined with magnesium and iron in the chloritic 

 mineral which is present. 



Finally, it is worth recording that in the three (Kilbride, St. 

 David's, and M[ena) areas, the three similar rocks are all accompanied 

 by a porphyrite. In the Kilbride area there is no direct evidence 

 of transgression to prove w^hich rock was the earlier, but the 

 occurrence in the field suggests that the porphyrite was the later of 

 the two. In the Msena area Prof. W. C. Brogger finds bostonites 

 and camptonites as complementary rocks, that is, diflferentiation- 

 products of one and the same magma. ^ Although no camptonites 

 occur in the Kilbride area, it is interesting to note that we have 

 found some intrusions of hornblende-lamprophyre in the Arenig 

 rocks of Tourmakeady.- 



(c) The Labradorite-Porphyrite. 



This rock is very constant in character. The extensive fine- 

 grained ground-mass is always purplish brown in colour, and is 

 crowded with large fresh, pale-grey, platy crystals of labradorite, 

 having an average length of about 5 millimetres and a breadth of 

 about 1 mm. Their outline is commonly somewhat rounded. The 

 specific gravity of this rock is 2'72. The ground-mass is seen in 

 section to be principally composed of felspar-laths, which are less 

 fresh than the phenocrysts and give a smaller extinction-angle. 

 Presh augite sometimes (85) occurs, and as a rule chloritic and other 

 pseudomorphs, apparently after rhombic pyroxene, are present. 

 Yery small vesicles filled with chlorite and chalcedony are of 

 frequent occurrence. 



{d) The Dolerites. 



These include mica-dolerites, in addition to the ordinary augite- 

 dolerites. 



(i) The augite-dolerites.— These are dark or nearly black 

 rocks, of medium grain as a rule, the texture in the larger dykes 

 becoming finer towards the margin. In some hand-specimens no 

 crystals are apparent ; but in some cases (58, 62) large augites are 

 seen, and in others porphyritic felspars (60, (52). Pyrite is oc- 

 casionally seen, and small calcite-fiUed vesicles are not uncommon. 

 The average specific gravity of sixteen specimens was 2*82. 



In section, most of the rocks are seen to be normal dolerites — 

 consisting of a felted mass of plagioclase-laths with augite and mag- 

 netite. Xo olivine has been definitely recognized ; but pseudomorphs 

 in carbonate, perhaps after olivine, are occasionally met with, as, 

 for instance, in the mica-dolerite (66). The size of the felspar- 

 laths varies a good deal, being generally greater in the dykes in the 

 south-western part of the area with a south-easterly trend, than 

 in the sills associated with the Calcareous Flags. These dykes 

 also tend to be fresher than the sills. 



1 W. C. Brogger, Q. J. G. S. vol. 1 (1894) p. 26. 



2 C. I. Gardiner & S. H. Eeynold?, Q. J. G. S. vol. Ixv (1909) p. 131. 



