Gh. X.] ON THE PRIMARY ROCKS. 207 



granite is felspar, which averages at least three quarters of 

 the whole mass, either in a crystalline form or in that state 

 called compact felspar, large beds of which frequently alter- 

 nate with common granite. This compact felspar is of two 

 kinds : the one massive and crystalline, the other also massive, 

 but with a fine granular and somewhat earthy texture ; the 

 first predominates in the granite of the western, the second 

 in that of the eastern districts. So likewise in the western 

 schistose group, various felspar porphyries and similar rocks 

 are interstratified with slates, having a basis of compact fel- 

 spar ; and in the eastern district, several species of eurite, 

 analogous to those in the adjoining granite, are associated 

 with slates which have bases of the same kind of earthy gra- 

 nular compact felspar. In the primary district of the eastern 

 part of Ireland, the granite of the northern part is micaceous 

 with a frequent predominance of quartz; and the adjacent 

 schistose rocks are different kinds of mica-slate, passing into 

 and alternating with quartz- rock: on the other hand, the 

 granite of the western part is principally composed of felspar 

 and mica, " the quartz, comparatively speaking, seldom ap- 

 pears, and the mica often passes into chlorite or hornblende, 

 all of which are not unfrequently intimately united with the 

 felspar into a homogeneous compound." The adjoining slates 

 here, also, exhibit a similar change of characters, having 

 an uniform texture, like the rock commonly called clay- 

 slate; and passing into hornblende-rocks, with which and 

 with quartz-rock they experience frequent alternations. 

 The fine granular basis in this and in the granitic group 

 may be owing to a blending together of the felspar and 

 quartz, as in the eurites of Cornwall, and thence the quartz 

 cannot be distinguished in the granite, though it is some- 

 times developed, as in the slate, in the form of beds of quartz- 

 rock, by its predominance over the other minerals. In 

 Scotland, we have seen that the characteristic mineral of the 

 granite is hornblende, which, like the shorl of Cornwall, 

 occasionally usurps the place of the mica altogether ; quartz, 



