Coir & CunnincHam—On Certain Rocks styled ‘“‘Felstones.” 321 
traced at short intervals through several fields on the other side of 
the road. The lower slopes of the promontory are covered by 
deep glacial drift, but apparently the same dyke reappears upon 
the shore. Through the kindness of the officers of the Geological 
Survey of Ireland, we have been enabled to consult the manuscript 
six-inch map, on which the surveyor of the district has marked a 
“rotten dyke ?Gabbro” to the south of this point. This may 
possibly refer to the mags that we are now describing. The whole 
length of the Drumboy dyke is a little over halfa mile. At the 
highest point, where the rock is well exposed, the structural planes 
of the coarse micaceous grit dip at an angle of 23° towards a point 
10° west of north. The dyke cuts through these, dipping at 
about 30° in a direction 40° south of east, and is thus clearly 
posterior and intrusive (Pl. XX.). 
The rock is vesicular throughout, the hollows being lined with 
red soda-orthoclase ; subsequently, calcite has often been deposited 
in the central part of the cavities. The mass is pinkish or greyish 
brown, and is a good deal decomposed. In microscopic section, 
the felspathic constituent is seen to be too greatly altered for 
determination by specific gravity; but the large number of 
untwinned or simply twinned crystals indicates the presence of 
orthoclase or anorthoclase. The repeatedly twinned felspar is 
a basic andesine; but the groundmass as a whole gives a flame- 
reaction indicating some five per cent. of potash. The same 
association of felspars is recorded by Mr. Flett in the camptonite 
of Rennibuster near Kirkwall.! The other mineral uniformly 
scattered through the groundmass is green hornblende, yellowish in 
cross-section, and occurring in small elongated prisms. Magnetite 
is present in octahedra, altering to hematite, and giving rise 
to “‘martite’’ pseudomorphs. Crystals of titanic iron oxide, 
probably titaniferous magnetite, now altered for the most part 
into leucoxene and translucent sphene, play the part of a porphyritic 
constituent, but do not measure more than °2 of a millimetre in 
diameter. They are clustered together in little groups, especially 
in the outer regions of the dyke. 
Near the margin, the constituents diminish in size, and the 
rock becomes more and more compact. A few felspar crystals stand 
1 Op. cit., Trans. R. Soc. Edin., vol. xxxix., pp. 876 and 887. 
2A 2 
