County Dublin, Past and Present. 



55 



abundance of quartz which, when the rock decays away, goes 

 to form sand in the river- valleys and on the shore. 



Other minerals are known in the granite of the Dublin 

 Mountains/ and a second species of felspar is practically 

 always present, ranging from Albite,^ as highly silicated as 

 orthoclase, but with soda in place of potash, to Oligoclase,^ the 

 soda-lime felspar. For references to many previous papers and 

 for an admirable series of investigations into the characters 

 of the Dublin granite, the reader must turn to the memoir by 

 Prof. Sollas just cited, which is at present the text-book of 

 the subject. We must content ourselves with quoting Dr. 

 Haughton's'^ average analysis of (a) eleven specimens taken at 

 intervals along the lycinster chain, and his deduction (b) as to 

 the mean mineral constitution of the rock, in parts by weight. 



B 



Silica 



Alumina 



Iron peroxide 



Lime 



Magnesia 



Potash 



Soda 



Loss by ignition 



The orthoclase, as the lightest mineral in the rock, naturally 

 plays a much greater part in the total volume than the 15 J per 

 cent, assigned to it in the above mode of calculation. If the 

 lime of the so called ** paste " were present as a constituent of 

 oligoclase felspar, a slight readjustment of the figures would 

 be necessary. 



The chemical constituents of these minerals were clearly 

 once fused together ; on the cooling of the intrusive mass, the 

 micas separated out first, then the felspars, and finally the 

 residual silica, as quartz. But experiment shows that this 

 complete and often coarse crystallisation is only possible 

 under pressure of overlying rocks and very slow conditions of 

 cooling. When the materials of a granite are erupted at the 

 surface as a molten lava, they cool as a black glass. Obsidian, 

 with bands and patches of imperfectly developed cr>^stals ; such 

 products are well known in lyipari and in Iceland. But in Co. 

 Dublin the molten rock appears nowhere to have reached the 



HVeaver, Trans. Geol. Soc. London, v. (1819), 135; Galbraith "On 

 Killinite," G. S. D., vi., 165; Haughton "The Mineralogy of the 

 Counties of Dublin and Wicklow," G. S. /., v., 43 ; O'Reilly, "On Micro- 

 cline Feldspar in the Dalkey Granites," 77>id., v., 189; O'Reilly, " On Beryl 

 in Glencullen," tdid, vii., 69; Joly, "Beryl and lolite of Glencullen," 

 Froc. R. Dublin Soc. (new series,) v., 48. 



2 Haughton, "Albite in the Granite of Dalkey," G. S. I., ii., 213. 



3 Sollas, "Contributions to a knowledge of the Granite of Leinster," 

 Trans. R. Irish Acad., xxix. (1891), 452. 



G. S. /., V. (1878), pp. 41 and 43. 



