328 



mountain of Krabla* 1 * in Iceland, with the augitic granophyres of 

 Carrock Fell in Cumberland and with the rock of Ennerdale above 

 referred to. Various sections in the British Museum, prepared from 

 specimens collected by Professor JUDD, agree with the above 

 so that there is every reason to believe that augitic granophyres 

 are largely developed in this island. 



Granite is extensively developed in the northern portion of the Isle 

 of Arran. It is a typical granite, or rather granitite, and differs, therefore, 

 from most of the rocks above referred to. Two varieties are recognized : 

 the one coarse-grained, the other fine-grained. Veins of the latter 

 occur in the former. Both are grey in colour. The coarse-grained 

 variety consists, according to ZIRKEL, of orthoclase, plagioclase, quartz 

 and a brownish black mica. Drusy cavities containing white orthoclase 

 and smoky quartz are common. The fine-grained variety is similar 

 in composition. Mica is, however, only sparingly developed and quartz 

 is very abundant. Sections of the Arran granites in the British 

 Museum show that the quartz is often idiomorphic with respect to 

 felspar, and that there is sometimes a tendency to a micro-pegmatitic 

 structure. Indeed the relation of the quartz to the felspar is similar 

 to that of the Dimetian granites. The closest relation exists between 

 the granites of Arran and those of the Mourne Mountains. This is 

 shown not only in the nature and relations of the constituents of the 

 main mass but also in the occurrence in both of drusy cavities 

 containing orthoclase and smoky quartz. 



South-east of Ireland. The granites of the main chain are fairly 

 uniform in composition. They consist essentially of two felspars, two 

 micas and quartz/ 2 * The accessory minerals mentioned by Dr. HAUGHTON 

 are tourmaline, beryl, apatite, garnet, fluor, and spodumene. Mr. JOLY 

 has recently detected cordierite in the Glencullen quarries/ 3 * 



Orthoclase occasionally attains large dimensions and the rock then 

 becomes a porphyritic granite. The ground-mass of the porphyritic 

 granite not seldom assumes a pegmatitic or granophyric stru cture according 

 to VON LASAULX. Veins of a fine-grained eurite often traverse the main 

 mass. These are sometimes devoid of any micaceous constituent. A 

 secondary schistosity has been developed at the margins of the main mass 

 in certain localities. This maybe seen in the neighbourhood of Killiney and 

 Dalkey. The macroscopic and microscopic characters of the schistose rocks 

 are similar to those of the Porthalla " gneiss " above referred to, and a de- 

 scription of the rocks would involve a repetition of what has been already 

 said in speaking of that rock (See Plate XLIL). Some of the rocks in which 

 secondary schistosity has been produced were originally micro-pegmatitic 

 and traces of this texture may still be recognized between the planes along 



(1) R. BREON. Geologie de 1'Island, Paris, 1884. 



(2) Dr. HAUGHTON. Q.J.G.S., Vol. XII., p. 171. 



Prof. VON LASAULX. T.M.M., Neue Folge. Band I., p. 433, 



(3) Proo. Roy. Dublin Soc. 1885., p. 48. 



