﻿268 ME. C. I. GARDINER AND PROF. S. H. REYNOLDS ON [May 1 9 10, 



Y. PETRO GRAPHICAL DETAILS. 



Owing to the close resemblance which the Glensaul rocks bear to 

 those of Tourmakeady, which we have already described, a brief 

 reference to their principal characteristics will here suffice. 



(a) The Felsites. 



(1) The main felsite mass. — The rocks of this mass are 

 everywhere singularly uniform in character. In a hand-specimen 

 they are brown in colour, and as a rule show prominent quartz 

 phenocrysts. The specific gravity of eleven of these rocks was 

 taken, and ranged from 2*64 to 2*71, the average being 2*68. The 

 ground-mass is always imperfectly spherulitic, and the large and 

 numerous quartz phenocrysts are strongly corroded by it. The 

 other minerals present are felspar, in a much weathered state ; 

 rhombic pyroxene, replaced by green pseudomorphs ; and mag- 

 netite. The presence of the two last-named minerals in considerable 

 quantities is the most noteworthy feature of the rock. 



(2) The small felsite intrusions. — The only one of these 

 that requires any special description is the peculiar nodular rock 

 from near the boundary of the main felsite mass due south of 

 Garranagerra. This rock is exactly similar to that from stream F, 

 in the Tourmakeady district, described on p. 134 of our previous 

 paper. The nodules consist of white quartz. 



The other small intrusions resemble the main mass in petro- 

 graphical characters. 



(b) The Tuffs. 



These may be divided into : (1) tuffs belonging to the Shangort 

 and Tourmakeady Beds, and (2) tuffs belonging to the Mount 

 Partry Beds. 



(1) Tuffs belonging to the Shangort and Tourmakeady 

 Beds. — These include (a) fine gritty tuffs; (/3) limestone breccias; 

 and (y) coarse felsite breccias. 



(a) The constituent elements of the fine tuffs are : — 



(1) Lapilli, commonly of felsite, which often show a semi- 

 spherulitic character. 



(3) Broken Mspar crystals. } also derived from the felsite ' 



(4) A calcareous matrix. 



These elements are variously combined in different varieties of 

 tuff, but the prevalent type consists principally of felsite lapilli and 

 quartz grains, sometimes with a little calcareous matrix. By the 

 increase in the proportion of quartz grains the gritty tuffs pass 

 gradually into grits, and by the increase in the calcareous matrix 

 into ashy or gritty limestones. 



