COMPACT FELSPAR. 155 



Aberdeenshire, remarkable for the destruction of all 

 its rocks, and for the great consequent depth of its 

 u ntran sported alluvia. Though the indestructibility 

 of the granitic varieties of gneiss points it out as ad- 

 apted to the purposes of architecture, the difficulty of 

 shaping it almost precludes its application to building. 

 But, in the micaceous varieties, this objection does 

 not hold; while considerable labour is saved by the 

 possession of, at least, two natural parallel surfaces. 

 When fissile, it thus also becomes fit for roofing; par- 

 ticularly in those mountainous districts where slates 

 of ordinary weight are unable to preserve their places. 

 In Scotland, it contains metallic veins, producing 

 lead and copper, and, rarely, plumbago : as it 

 includes tin, on the continent of Europe ; where it 

 appears to abound more in metalliferous veins than in 

 this country. It there also produces copper, silver, 

 antimony, zinc, wolfram, lead, molybdena, and gold ; 

 but the last two substances, in particular, are rare. 



Compact Felspar. 



That compact felspar which occurs among the por- 

 phyritic and trap rocks, forming the basis of porphyries, 

 is well known to geologists. But I cannot discover 

 that a similar, if not the same mineral, has been ob- 

 served as a member of the primary strata, unconnected 

 with any masses of porphyry, or of the other intruding 

 rocks. It is therefore a new rock in the system, in 

 addition to all the others which I have introduced ; 

 while the reasons for associating it in the same 

 chapter with gneiss, will be apparent. 



The term compact felspar is here therefore under- 

 stood to mean a stratified rock, if not always very re- 

 gularly so, forming a member of the primary strata, 



