138 



Proceedings of the Royal Irish Academy. 



size, and well-rounded — for the most part consisting of coarse granite 

 — and sometimes they are crushed beyond recognition, as in the 

 peninsula between Cleggan Eay and Ballynaskill Harbour, in North 

 Galway. The black slate is also fairly persistent, usually below, 

 sometimes above the limestone, and occasionally containing pebbly 

 seams and boulders, as at Keem and Doogort, in Achill, respectively. 



Accounts of the views to which I have been led during the 

 revision-work, embodied in two papers,^ were submitted to the 

 British Association at its Glasgow meeting in 1901, in which 

 references were made to the order and mode of occurrence of the 

 different formations represented in Mayo and Galway; but the 

 accounts were necessarily very brief. Mr. McHenry has set forth 

 his views regarding the Ox Mountains in a paper read before the 

 Academy, already alluded to in this description. The present account 

 is intended to supply important details met with in the course of my 

 own work, and not previously published in connected form. They may 

 furnish some aid in the formation of a future set of maps of this 

 exceedingly interesting and instructive region. The value and 

 bearing of the details may be judged from the following connected 

 account : — 



Metamoephic Series. North Mayo. 



The coarse gneisses of Erris Head, as we have seen, are not the 

 most ancient metamorphic rocks ; on the contrary, they are but 

 crushed, coarse-grained granites, or pegmatites which have been 



N 



(!,3) (tn) (200) iro> 

















1 "^-^s^ 



Fig. 2. — View of Erris Head (N) and section southward, showing coarse pegma- 

 titic gneiss (G), mica-schist {m), pebbly deposit (i), and quartzite (q). 



intruded into mica-schists, immediately underlying the great quartzite 

 group, well represented in North Mayo. Thus if a section be taken 

 three quarters of a mile west of Erris Head, running south -south-west, 



1 Report for 1901 : "On the Eelation of the Silurian and Ordivician Rocks of 

 the North- "West of Ireland to the great Metamorphic Series," by J. R. Kilroe and 

 A. McHenry, p. 636 ; and On the Relations of the Old Red Sandstone of Nortii- 

 West Ireland to tlie adjacent Metamorphic RocLs, and the Torridon Rocks of 

 Scotland," by A. McHenry and J. R. Kilroe, p. 636. 



