478 1'p.f. T. <;. 15 ..... Btml&n tend ot) - men 



<lc\cl'. [><! (-ee figure). Some of the larger diallagi- crystal- aU. 



a mi ii, ii.- structure with crossed nicol> ; small lamelln- of a different 



tint, arranged in a kind of network with lo/fn-c-.-ha|ed nie.-hc-. making 



Pegmatitic association of a pyroxene and felspar (composite), x 21. The 

 "rootlets" and most of the mineral round the central grain is pvroxene. 

 Decomposition shown about a crack.* 



their appearance. These possibly may indicate an early stage of the 

 conversion of the diallage into hornblende. The grains of felspar vary 

 much in size, even when associated with the " rootlets " of pyroxene. 

 They are generally in good preservation ; exhibit twinning, usually on 

 the alliite type, and are shown by the extinction angles to be nitly if not 

 wholly labradorite. Small grains of iron oxide are present, which are 

 most abundant near the margin of the larger pyroxenic grains. They 

 arc .-oinetinie.- scattered in the pegmatite, and in one or two cas< 

 slightly root-like in shape. Cracks traverse the rock and Imve led to 

 mineral change. They are often lined with small crystals of a In-own 

 mica, similar to that which occurs in some specimens of the "blue 

 ground." These are iml>edded in a rather earthy-looking granular 

 material, which is, no doubt, a decomposition product from the felspar. 

 Pegmatitic .structures, whether macroscopic or microscopic, are fairly 

 common in granites, where the associated minerals are quartz and felspar, 

 but, so far as my experience goes, are infrequent with other minerals. 



* I am indebted to my friend Mr. C'oo:uara-Swaiuy tor the microphotograph. 



