118 



Proceedivga of the Roi/al Irish Acaihm.fi. 



amphibolic material into the diabase. On either side of the vein, prisms of 

 green amphibole have shot out at right angles to its walls. These have now 

 become chloritic, like the groundmass. The adjacent rock has become 

 partially foliated, and has assumed the shimmer of a fine-grained hornblende- 

 schist; but it is difficult to suggest pressure as the cause of so local a 

 phenomenon. The felspathic constituent of the diabase has disappeared ; and 

 the rock is a dense mass of granules of pyroxene, abundant tufts of chlorite, 

 brown mica, and a trace of colourless matter ; here and there a crystal of 

 amphibole passes across it like a blade. The rock has been darl^ened on 

 either side of the vein by an exceptional development of brown mica. 



Fig. 



-Tliiii stiui.ii ui juiiil-inick coiivi-ilcl into u iiiiiiuiul >eiii, with rliloiiliauii ampliib.-n; 

 pviiulrutiii){ llie diabusv %i\\ vilLur side. riclurc-Uuuk, Co. Donegal, x II. 



Sections of the spheroids show how, even here, the felspar crystals liave been 

 entirely changed. They can be seen as small white rods with the naked eye, 

 but are now composed of minute prisms of zoisite, lying in all directions, an 

 occasional gi-aiiule of epidote, and chlorite. This chlorite must be an impor- 

 tation from the groundmass, in which it is abundant, togetlier wiih lirown 

 mica, probably as the product of the alteration of granular pyroxene, as well 

 as of undiHerentiated glassy matter. Dr. J. S. Hylaud, in his very accurate 

 notes on the petrography of Donegal,' states that the felspars in the 

 epidiorites of the region rarely retain traces of twin-structure, and have 



' Geol. Surv. Ireland, Mem. Ui sheets 3, \, Inn, (Itt&l), p. 133. 



