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dark brown mica. Both the felspar and hornblende of this rock have been 

 analysed. The rock forms part of a large mass (3 by 14 miles) which is 

 stated by Dr. HEDDLE to vary greatly in composition. In some portions of 

 the mass the principal ferro-magnesian constituent is augite, in other 

 portions hornblende and in others biotite. This mass forms the " syenite " 

 of Hay Cunningham. Labradorite appears to be the dominant felspar. 

 Dr. HEDDLE also describes a labradorite-hornblende rock occurring on the 

 north side of Glen Bucket in which the crystals of hornblende occa- 

 sionally attain gigantic dimensions (21 inches in length by 2 in breadth) 

 and an anorthite-hornblende rock from Trista in Fetlar, one of the 

 islands of the Shetland group 



In dealing with the plagioclase-hornblende rocks the artificial character 

 of our system of classification is admirably illustrated. Quartz-diorites 

 of intermediate composition shade into basic plagioclase-hornblende rocks 

 by the most insensible gradations. Some of the more crystalline rocks 

 from the Assynt district of Sutherland which we shall describe in the next 

 chapter because they are associated with normal hornblende- porphy rites, 

 contain a smaller amount of Si0 2 (52 to 54 p. c.) than the rocks already 

 described as andesitic dolerites. 



