108 Proceedings of the Royal Irish Academy. 



the structural planes.*^ Gradually the rock becomes typical schist, when 

 in some of the micasyte, talco-micasyte, and talc-yte, a peculiar 

 frilled or sharply crumpled struction is developed. This structure 

 is very remarkable, as no structure similar to it occurs in the unaltered 

 rocks. Moreover, it seems to disappear in the last stage of meta- 

 morphism as it has not been remarked in any of the rocks of the gneiss- 

 series. In some of the quartzytes or quartz-schist there is also a 

 peculiar structure, having an aspect scarcely distinguishable from the 

 planed, polished, and etched surfaces due to ice action. This, how- 

 ever, always occiu's only on the original planes of stratification, and 

 if the rock is flaggy and is split into plates the surface of each plate 

 will be a counterpart of that above it, from which it would appear this 

 structure is probably due to a cleavage in the original rock. If this 

 was the case all other traces' of the cl( avage have now disappeaied, as 

 the present rock has no tendency to split except along the planes of 

 deposition. 



The schists graduate into gneiss and the gceiss through granitoid- 

 gneiss or gneissoid-grauite into typical granite. 



The granitoid- gneiss has a foliation peculiar to itself. In all 

 the other rocks belonging to both the schist and the gneiss series the 

 foliation appears always to follow the most prominent structure in the 

 original rock, let it be fine- jointing, cleavage, or lamination, whether 

 the latter is parallel, oblique, cmied, spheroidal, concretionary, 

 nodular, or conglomeritic.f In granitoid-gneiss other laws seem 



* The partially altered grits and sandstone called by MaccuUocli " Primary 

 Sandstone," {A Geological Classification of Rocks, ^-c, by John Macenllocb, M. D', 

 F. E. S., &c., page 331), are largely represented in the country west of the northern 

 end of Longb Mask. Associated with them are subordinate beds of micasyte 

 argiUyte and clay-slate, while towards the west they graduate into quartzj-te. The 

 clay-slate and argillyte are due to the metamorphismof argillaceous rocks, while in this 

 locality much of the micasyte was originally a micaceous, obliqiiely laminated 

 sandstone, and in the schist it is quite apparent that the foliation has been induced 

 by the oblique lamination. The " Primaly Sandstones" are, for the most part, mas- 

 sive, compact, and hard, generally very arinaceous, but sometimes felsitic ; rarely 

 are they gneissoid. As they graduate into quartz3d:e a fissile structure is developed 

 parallel to the stratification planes ; they are general!}'- of a reddish or pirrplish 

 coloiu\ 



t The frilled or sharply crumpled structiu'e jjrevionsly mentioned, in some 

 niicas}i;e and talcytes, is the only exception to this general rule that I have detected. 

 A crumpled lamination occurs in some unaltered argillaceous rocks, to which, 

 in another place, I di-ew attention {On Crumpled Lamination in Shales, Journal Geol. 

 Soc, Dublin, 1863) ; but that has its representative among the foliation. This 

 also I have previously pointed out {Xotcs on the Foliation of the gneiss and 

 Schist of Yar-Connaught, Journal Royal Geol. Soc, Dublin, 1866). Moreover, it has 

 not an aspect analogous to this structure ; furthermore, it is rare, while this is not 

 uncommon. In schistose-Hmestone, schistose-dolom3i;e, and ophj-te, there are very 

 complicated foliations, and at one time I was inclined to suppose they miist be due- 

 entii-ely to the metamoi'phic action. This, however, I find not to be the case, for, on 

 a careful examination in many places of unaltered limestone, I find that, as a general 

 rule, it is always more or less affected by aU sorts of iiTegular sti-uctures not 



