DIFFERENTIATION PHENOMENA OF THE PROSPECT INTRUSION, 133 



(1) an increasing percentage of albite and orthoclase, 



from nil to 77*56%. 



(2) a decreasing percentage of labradorite,from 37*2 to nil 



(3) a progressive change in the pyroxenes, the augite 



being replaced by diopside and segerite, the total 

 percentage decreasing from 40*5% to 17*47%. 



(4) a decreasing percentage of ilmenite. 



(5) a total absence of olivine in the segregation veins. 



Chemically considered the variation is as shown in the 



following table: — 



(1) Essexite. < 2 > Ess ° X °\ ^ Es f.f°- 

 x ' 'pegmatite. aplite. 



Increasing acidity Si0 2 41-05 ... 58-82 



„ alkalinity K 2 + Na 3 296 8-08 970 



fCaO 10-96 ... 2-42 



Decrease of Bases J FeO 11-07 .... 4-59 



(MgO 6-38 . . 088 



It will be seen from Table II, that the essexo-pegmatite 

 is intermediated in its composition between the essexite 

 and the essexo-aplite. The coarse variety of pegmatite is 

 somewhat more acid than the fine grained variety. An 

 examination of figure 2 will show that the order of forma- 

 tion of these rocks was as follws : — 



1. Essexite. 



2. Fine essexo-pegmatite. 



3. Coarse essexo-pegmatite. 



4. Fine essexo-aplite. 



5. Coarse essexo-aplite. 



The pegmatite, when it occurs in the same vein with 

 the aplites, always occurs at the sides of the veins and 

 merges by imperceptible gradation into the essexite proper, 

 the coarser and finer types also merge into one another ; 

 the junction between the pegmatite and aplite is always 

 fairly sharp. There is no real textural difference between 



1 No complete chemical analysis of the pegmatite is available. 



