has noted in the allivalites of Rum that in anorthite-olivine 

 rocks, when the anorthite is in excess, it has crystallised 

 first. 1 Experimental work, according to Pratt and Lewis, 

 shows the same order. To account for those rocks in 

 which pleonaste occurs without granophyric intergrowth 

 we may perhaps suppose that they were completely 

 remelted during the addition of peridotitic material and 

 recrystallised in the more normal order, pleonaste being, as 

 usual, the first mineral to form. But it must be noted that 

 in these rocks, so far as yet studied, olivine does not occur. 

 How far this may be a determining cause of the absence of 

 the granophyric intergrowth is not clear. 



It will be noted that the explanation here advanced for 

 the origin of pleonaste is analogous to that offered for the 

 occurrence of kelyphitic rings about olivine. Pratt and 

 Lewis show that about olivine crystals in olivine felspar 

 rocks there is sometimes "a three ply development, the 

 layers, beginning next to the olivine, are as follows: — (a) 

 enstatite... (b) diopside... (c) actinolite... Everywhere 

 except in a thin layer next the anorthite the actinolite is 

 intimately intergrown with irregular masses and tangled 

 vermiculate stringers of pleonaste."' 2 Dr. Adams, they say, 

 regards the reaction rims as due to the interaction between 

 olivine and the still molten plagioclose, but the authors 

 cited suggest that the felspar may be the earlier. "Assum- 

 ing that these rocks are intrusive masses, and that the 

 anorthite represents a deep seated crystallisation, sufficient 

 change in condition might have been produced by their 

 intrusion into their present position to have caused the 

 corrosion before cooling. This reaction would be quickly 

 stopped, however, by the solidification of the rock, The 

 products resulting from the combination of the i 

 1 Natural History of Igneous Rocks, p. 



