On the Physical Geology of the Neighbourhood of Dublin. 148 
The mineral constituents are (1) Quartz, (2) Orthoclase fel- 
spar in distinct crystals, (3) Albite felspar (in paste), (4) White 
mica (Margarodite), (5) Black mica (Lepidomelane). The quartz, 
orthoclase, and white mica, are always present in distinct 
grains; the black mica is not always, but frequently, present 
along with the white, and in smaller proportion. ‘The ortho- 
clase crystals are sometimes large, making the rock porphyritic. 
The felspar paste contains much more soda than the ortho- 
clase crystals. This suggested the idea to Sir Robert Kane, 
Dr. Haughton, and others, that the paste might be partly com- 
posed of some other felspar besides orthoclase. None such, 
however, had been seen in this granite, until Dr. W. H. Stacpoole 
Westropp detected what seemed to him to be some small crystals 
of albite in some granite from the neighbourhood of Kingstown. 
These proved to be really albite on being analysed by Dr. Haugh- 
ton. Since then, Professor Hull has, by means of the microscope, 
observed in the paste of this granite, besides orthoclase, a triclinic 
felspar, which is doubtless albite. (In the Mourne granite the 
albite, as well as the orthoclase, can be distinguished in every 
hand specimen.) The white mica of the Leinster granite some- 
times becomes plumose, At different places about Killiney and 
at Foxrock, near Carrickmines, it has been found collected into 
nests, with a beautiful flowing, feathery arrangement. It is 
believed that this is a speciality of this neighbourhood. 
For the accidental minerals occurring in this granite and de- 
veloped: in the Silurian rocks pain hom taned thereby, see the 
Article on the mineralogy of this district. 
The jointing of the granite can be well studied in many places, 
as about the Killiney Hills, especially in the large, now disused, 
quarries on Dalkey Hill, Several joint Bh different degrees 
of importance can sometimes be seen intersecting at the same 
place. The joint surfaces are almost always very even and 
smooth, entirely different, as to character, from surfaces of frac- 
ture; so that however the joints may be connected with the 
contraction of the granite in cooling, they are something more 
than mere planes of splitting. The main joints have generally a 
marked parallelism, sometimes for considerable distances, 
It is principally the primary joints, and rarely the others, that 
have the slickenside coatings so often to be seen on those surfaces, 
