222 .^- ^oTô. 



mortur structure, or with tlie same, a micropeginatic iiitei-growth; 

 the latter is developed on the peripheral zones of other feldspars, and 

 the stalks of quartz, which are disposed in regular orders, stand at 

 riofht ano'les to the faces of the enclosed mineral. 



The plagiüclase occurs in the fresh state. Muscovite is always 

 the mineral formed from it when weatliered ; no epidote crystal is 

 ever found, a fact which speaks in favour of the acid character of 

 the feldspar, of its Ijeing poor in lime. The quantity of the 

 plagioclase is greater than that of the orthoclase and microcline. The 

 plagioclase is usually of a large size, and with quartz it forms a part of 

 the micropegmatic intergrowth. The twinning lamellation is distinct- 

 ly developed, yet not so finely as in that of the laljradorite series. 

 Well-outlined clinopinacoids are seldom met with, as is usually the 

 case in such granular rocks as granites, and the direction of extinction 

 could be very nirely measured on that face ; some opportunities were, 

 however, afforded for such an observation, and it then extinguished 

 light in tlie positive sense at about S'' with the edge I'/M. ^Sections with 

 twinning lamellations make an extinction-angle of 4^ symmetrically 

 with reference to the trace of the twinning sutures. From this it 

 may be inferred that the plagioclase in question is of an oligoclastic 

 nature, or else is closely allied to oligoclase, perhaps albite. A feature 

 which characterizes this variety of feldspar is its zonal structure, and the 

 presence of this feldspar could be readily recognized by the zoning, even 

 without examiiiing for the existence of the polysynthetic twinning. 

 The plagioclase is rich in crystalloids of hornblende, lamelke of biotite, 

 round-edged needles of apatite, and nearly ellipsoidal crystals of zircon. 



J'he quartz occurs, as is usually the case in granites, in a xenomor- 

 phic, angular form. Macroscopically it seems n(jt to have taken an 

 inqxjrtant part in the composition of the rock, and in some varieties it 

 sinks into so small a quantity as to give to the specimen the appearance 



