On the Physical Geology of the Neighbourhood of Dublin. 143 



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. fsince then, Professor Hull has, by means of the microscope, 

 observed in the paste of this granite/besides orthoclase, a triclinfc 

 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 metamorphosed 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 systems of 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. 



