106 Proceedings of the Royal Irish Academy. 



occTUTing in a light purplish, blue matrix ; these maculae decompose 

 freely, and give the weathered surface of the rock an irregular honey- 

 comb aspect ; they also fuse easily before the blowpipe, but the matrix 

 only slightly on the edges of splinters. There is also a variolitic green- 

 ish variety, dark green small concretions in a light green base ; this 

 rock is very friable and possibly may be a compact tuif and not a nor- 

 mal igneous rock. All these felstones are basic varieties, the eurytes of 

 Daubuisson.* In some of the dykes of rnicaceous-euryte there is a fine 

 platy structure that unless carefully examined may be mistaken for 

 foliation. In different places in this area, but very numerous in the 

 west part of the barony of Ballynahinch, are dykes of basic-elvanyte ; 

 these in places seem to graduate into these eiuytes, and probably were 

 the roots or deep-seated portions of the upper silurian felstones. 



Pre-LIandovery fehtones. — The rocks of this age only occur as dykes 

 and intrusive masses in the hypogene rocks. For the most part they 

 seem to 'hQ peiro-siJex or the rocks called /<?/s?/fe by Dana, Cotta, Forbes, 

 and others. Some are comoid, others saccharoid in appearance ; 

 usually the constituent are undeveloped, save the quartz, which appears 

 in some as blebs, globules, and crystal often very minute {quartzitic- 

 petro-silex). Some of the saccharoid vaiieties graduate into a rock very 

 similar in aspect to the fehitic-granite described hereafter. Either 

 kinds may become porphyiitic and micaceous, and through that graduate 

 into elvanyte. In some of the comoid petro-silex there is a riban of 

 different colours or of dark and light shades of one colour {irihan petro- 

 silex), while along the riban the rock has a tendency to split into 

 plates or slabs ; these rocks ai'e often quartzitic. A common variety of 

 petro-silex is of a whitish pink, greenish blue or gi'eyish colour, sub- 

 translucent, homogeneous, compact, breaking with a conch oidal fi'acture, 

 and weatheiing with a thick white crust. This variety, although com- 

 pact, is often effected by three or more systems of close jointing that 

 cause it to break up into angular shingle, often partaking more or less 

 of a cubical form. When not j ointed this variety weathers with a remark- 

 able smooth even surface. 



Camlro-silurian felstones . — In the country north of Killary Harbour, 

 associated with partially metamorjDhosed sedimentary rocks are fel- 

 stones (also in places partly altered) occuiTing in protrusions and in 

 dykes. The rock when unweathered is usually of a bright green 

 or grey colour ; it, however, weathers easily into a diity cream-coloured 

 rotten rock. In the dykes this felstone often has a pecxdiar breciated 

 structure having an appearance more like a coarse tuff or a fault-rock 

 than a normal felstone ; nevertheless, in other parts its true character is 



* Naumami and Cotta propose to call these varieties Torphyrites. It. however, 

 is a most inappropriate and confusing name ; for in the first place, as Cotta himself 

 points out, '■'■the name of porphyrite refers to a texture which is not an essential 

 feature of these rocks, because the porpht/rites are not always in fact porphi/ritic ;" 

 and in the second place, porphyrite and porphyry are used synonymously by many 

 petrologists, Cotta even so using it. 



