[ 263 ] 



XXIII. 



PEELIMINABY ACCOUNT OF THE SODA-GKANITES AND 

 ASSOCIATED DYKES OF CO. WICKLOW. By Professor 

 W. J. SOLLAS, D. So., LL.D. 



[Read January 9, 1889.] 



Since my detailed account of these rocks may not be ready for 

 publication for some little time to come, I propose on this occasion 

 to present to the Society a brief summary of the principal results 

 already obtained. 



Soda-Granite of Aughrim. — Just before the railway from Wooden 

 Bridge enters the town of Aughrim it passes through a cutting 

 which affords an excellent exposure of a fine-grained granite, strik- 

 ingly white, but speckled black with mica. A glance at a geolo- 

 gical map will show that this belongs to the same system of dyke- 

 like intrusions as the similar granite of the not distant Cushbawn 

 Hill, which lies a little to the north of Aughrim. The chemical 

 composition of the Aughrim granite, as shown by my analyses, 

 agrees very closely with that of the Cushbawn Hill rock, as deter- 

 mined by Dr. Haughton ; the former is, therefore, like the latter, 

 a soda-granite. Owing to the facility with which comparatively 

 un weathered specimens of the granite at Aughrim can he obtained, 

 I have selected it for a complete investigation, in preference to that 

 of Cushbawn Hill. 



The powdered rock, treated with a heavy fluid in a " Smeeth " 

 separator, yielded first magnetite, titaniferous magnetite, iron 

 pyrites, and zircon ; then followed a mineral, which I regard as 

 a species of epidote, having a specific gravity of from from 3-4 

 to 3*45 ; this was succeeded by biotite, which fell continuously 

 from 3*17 to 2*875, the denser portions having the composition of 

 Haughtonite ; with the biotite were associated a green mica and 

 chlorite — the latter a late stage in the alteration of the biotite, the 

 former possibly a first product of its decomposition. Between 

 2-875 and 2*71 muscovite fell, and at 2'56 this was accompanied 



SCIEN. PF.OC. K.D.S. VOL. VI., PT. V. X. 



