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Proceedings of the Royal Irish Academy. 



of ^'felstone" are shown traversing a basic Dalraclian rock at the 

 old stone fort in Cor, above the Gweebarra. These are in reality 

 formed of coarse aplitic granite, and stand out on the little hill like 

 two great walls. The basic rock is an amphibolite, seemingly free 

 from felspar, and with a specific gravity of 2*95. In its earliest con- 

 dition it was probably of greater specific gravity, and was a pyroxene 

 peridotite, since a number of pale green ovoid pseudomorphs, ophitically 

 embedded in the hornblende, and now largely composed of fibres of 

 actinolite, have the form and character of altered olivine (fig. 1). 



Fig. 1. — Section of amphibolite of Cor, Co. Donegal, showing traces of ophitic 

 structure. The pale spaces are occupied by actinolite, associated with 

 magnetite ; the fibres of actinolite are sometimes in continuity with 

 those of the darker hornblende round it. x 9. 



This rock broke through the schist-series, and includes blocks removed 

 from it. In turn it was invaded by the granite, which appears in 

 force on the neighbouring bare ridge of Trusklieve, and which comes 

 up in veins and patches through the Dalradian series all across the 

 intervening lowland. At the contact, the basic mass is distinctly 

 granitised," with frequent addition of pyi'ite. Sphene and epidote 

 appear in it, the latter at times including allanite. Allanite is also 

 present in brown independent grains. But the principal changes are 



