BY W. G. WOOLNOUGH. 505 



In most cases the felspcars are moulded on the biotite, but 

 occasionally there is an interpenetration of the marginal portions 

 of the two minerals, showing that the cr3^stallisation of the mica 

 had not completely ceased when that of the felspar commenced. 



Interpositions in the form of small prisms of colourless apatite 

 occur like those in the felspar. These do not give rise to pleo- 

 chroic halos. 



Hornblende, though less abundant than biotite, is ne\'ertheless 

 fairly plentiful. It occurs as completely allotrioraorphic granules, 

 usually independently developed, but in some cases intergrown 

 with the biotite. The colour in ordinary light varies from light 

 yellowish-green to strong green. Cleavage is quite normal, and 

 in addition the mineral is much shattered by the crushing of the 

 rock. The pleochroism Exhibits no points out of the common, 

 a = light greenish-yellow. 

 b = dark yellowish-green. 

 C = dark green. 



The absorption scheme being : it<b<C. 



The maximum extinction observed in the vertical zone was 22°. 

 Almost all the sections exhibit the usual (010) twinning. 



The crystallisation of hornblende has been almost synchronous 

 with that of biotite, and the two minerals are irregularly inter- 

 grown. In most cases the basal cleavage of the mica appears to 

 be parallel to the vertical axis of the hornblende, though an odd 

 section here and there does not seem to follow this rule. 



The alteration of hornblende is very similar to that of biotite, 

 and in patches, where complete alteration has taken place, it is 

 impossible to determine with certainty what the original mineral 

 was. 



The inclusions in the hornblende are so similar to those in the 

 biotite as to call for no special mention. 



A little magnetite is scattered through the sections, occurring 

 as inclusions in all the essential minerals indiscriminately. In 

 all cases it shows some trace of crystalline form. It is entirely 

 free from decomposition products, and the surface reflects incident 

 light brightly. 



