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XLYI. 



ON SOME SPHEEULITIC BOOKS FKOM CO. DOWN. By J. 

 SHEABSON HYLAND, Ph. D., M.A., of the Geological Survey 

 of Ireland. 



[Read February 10, 1890.] 



The district of trie Mourne Mountains has attracted the atten- 

 tion of numerous observers, including Berger, 1 Bryce, 2 Griffith, 3 

 Haughton, 4 and Hull. 5 Both in its physical features and geo- 

 logical structure it bears a striking resemblance to those of the 

 Island of Arran in Scotland. In petrographical characters there 

 is also the closest possible relation between the granites of the 

 two districts. This petrographical similarity may possibly be 

 considered an indication of contemporaneity. In both cases the 

 geological age of the granite is a point of conjecture, and is 

 incapable of actual demonstration. 6 



The granite of the Mourne Mountains is composed of two 

 felspars, quartz, and a dark mica. 7 It is, accordingly, a granitite 

 in Rosenbusch's sense. It often contains drusy cavities, in which 

 the constituents occur in a well crystallized form, and attain 

 considerable dimensions. The minerals observed in these cavities 

 are white orthoclase, albite, smoky quartz, beryl, chryso-beryl, 



1 On the Geological features of the N. E. Counties of Ireland, &c, Trans. Geol. 

 Soc, Lon., 1st series, vol. iii., p. 121. 



2 On the Geological Structure of the Counties of Down and Antrim, Rep. Brit. Ass., 

 1852, p. 42. 



3 Geological Map of Ireland, 1839. 



4 Experimental Researches on Granites of Ireland, Q. J. G. S., Lon., vols. xii. 

 and xiv. 



5 Report Brit. Ass. 1871, Trans, of Sections, p. 102 (with Traill.) 



6 E. Hull, in Memoir to accompany sheets 60, 61, and part of 71 of the Maps of the 

 Geological Survey of Ireland: Dublin, 1881, p. 9. 



7 Berger, I. c, p. 135, mentions amongst the "accidental ingredients" of the 

 Mourne granite crystallised hornblende and reddish garnet. The hornblende is chiefly 

 abundant in the porphyritic variety of the granite ; the garnet in the granular variety. 



