FELSPARS IN METAMORPHIC ROCKS. 361 



Mineral Constituents of Metamorphic Rocks. 



Any theory of relation between metamorphic rocks and plutonic 

 rocks must be based upon similarity in the composition of constituent 

 minerals in the rocks which are compared. Interesting researches 

 made by Professor Heddle show absolute correspondence between 

 the metamorphic and plutonic forms of minerals 1 in the same district. 

 And if any theoretical value is to be attached to the chemical com- 

 position of different rocks, the tables of composition of the chief 

 minerals which we reproduce will go far to show within what limits 

 a rock may vary in chemical composition owing to the proportions 

 in which its mineral elements are combined. 



Felspars. Orthoclase is found in gneiss at Glen Urquhart, Dee- 

 side, and central Sutherland associated with hyaline quartz, carrying 

 Lepidomelane or Haughtonite, and occasionally apatite. In chlorite 

 schist it is associated with chlorite and rutile. It is occasionally 

 found in porphyry, and exceptionally in syenite, as on Morven and 

 Froster Hills, and near New Leslie in Aberdeenshire associated with 

 hornblende, menaccanite, and sphene. More rarely it is found in 

 crystalline limestone. Orthoclase enters into a greater number of 

 rocks than any other felspar. 



Albite has only been found in Scotland in the red granite of 

 Peterhead, which contains hornblende and epidote ; and in another 

 hornblendic and serpentinous rock from Fetheland Point to Hillswick 

 Ness in Shetland. It forms the moonstone of Stromay. 



Anorthite or lime-felspar -is recorded in the diorite of Fetlar, and 

 some parts of the gabbro of Ayrshire, but is not a felspar found in 

 limestones. 



Latrobite, the lime-potash felspar, is found in the crystalline lime- 

 stone of Glen Gairn. 



Labradorite is nowhere found in Scotch metamorphic rocks, being 

 limited to gabbro, diallage rock, and dolerite. 



Oligoclase combined with hornblende forms the hornblendic gneiss 

 of Cape Wrath, and makes the bulk of the grey granite of Aberdeen, 

 where it is associated with a little orthoclase, a little quartz, a little 

 muscovite, and much Haughtonite, forming the compound termed 

 granitite. 



Andesine is in Scotland a characteristic felspar of gneiss, just as 

 oligoclase is characteristic of granite. Andesine is especially found 

 in limestone. 



These felspars are practically identified with the microscope, in 

 thin sections under polarised light, by the angle between the directions 

 of "Extinction" of adjacent laminae. When the angle exceeds 62 

 30' the felspar is Anorthite. If the angle is between 37 and 62 

 30' the felspar is probably Labradorite. If the angle is less than 37 

 it may be Oligoclase or Albite. 2 



1 M. F. Heddle, Mineralogy of Scotland. Trans. R. S. Edin., vols. xxviii. 

 and xxix. ; and Journ. of the Min. Soc., vols. ii., iii., iv. 

 - Fouque and Levy, Roches Eruptives 



