KiNAHAN — On Granitic and other Ingenite Rocks. 107 



indisputable. "WTien in protrusion a considerable portion of the mass 

 ■^ill be a scaly or mealy, friable, tuffoid rock ; some parts, however, 

 Tvill be compact, and all will be more or less unctuous as if the rock 

 was merging into steatyte. These rocks must be very ancient compared 

 with the rocks last described, as they were intruded prior to the meta- 

 morphism of the associated rocks. 



Htpogexe Roces. — The hypogene rocks include the metamorphic 

 and granitic rocks given in the general table. In the country west of 

 Loughs 3Iask and Corrib the development of these different rocks is 

 well shown, as noiTaal sedimentary, or derivate rocks first become 

 partially metamorphosed {schist series) ; secondly, the alteration becomes 

 more decided [gneiss series) ; and thii-dly, they are entirely changed 

 {metamorjMc and intrusive granite). Intervening between the 

 typical granites and the plutonic rocks are found the eiranytes. It 

 has previously been shown that the plutonic rocks graduate into 

 elvanyte and hereafter it will be pointed out that the latter rocks 

 graduate into the intrusive granite, and thi^ough that into the meta- 

 moi-phic granite. The rocks composing the schist-series are for the 

 most part schist ; nevertheless, in them are subordinate beds of gneiss, 

 on account of some rocks being more susceptible of change than others, 

 while in others the constituents of gneiss (quartz + mica + felspar or 

 felsite'^) already existed. The latter, however, are seldom typical 

 gneiss, that is, having these constituents arranged in leaves or plates, 

 but rather the minerals are more or less irregularly mixed together, 

 only an incipient foliation being developed. Similarly in the gneiss- 

 series there are subordinate beds of schist, as some rocks on account of 

 their constituents are less susceptible of change than others ; while 

 associated with the rocks of both the schist and gneiss-series are meta- 

 moi'phosed igneous rocks. 



When the rocks begin to be altered, the planes of the most marked 

 structure in the original rock are more or less glazed or micacised, and 

 the mass indurated. In argillaceous rocks the surfaces of joint lines, 

 breaks, or fissures are hardened and in some cases these, oiiginally open 

 lines, are sealed; or the sides, varying from a film to two or three inches 

 in width, are hardened, seemingly by being impregnated with some such 

 substance as silex in solution. On the surfaces of some rocks, crystal- 

 line lines about the size of whipcord form, sometimes straight but often 

 vermicular ; moreover, such minerals as phylite, chiastolite, pyrite, and 

 the like, are developed indiscriminately through the mass of the rock or 

 on the structural planes, being most conspicuous under the latter cir- 

 cumstance. A primary change, prevalent in arenaceous rocks, is a more 

 or less scaly or fissile structiure being developed, while others become 

 varioHtic or maculated, all being more or less indurated and glazed on 



* Fehite is the mineral, an amalgamate of felspar and eilex, while the rock will 

 be called Fehyte. 



