DIFFERENTIATION PHENOMENA OF THE PROSPECT INTRUSION. 12£ 



B. The Felspathic Phase of the Essexite. — This lies 

 between the pallio-essexite and the normal essexite and 

 has a thickness of about sixteen metres; it includes the 

 two larger segregation veins whose origin will be discussed 

 later. By reference to Table I, it will be seen that, as 

 compared with the normal essexite, it is notably richer in 

 felspars and biotite, and correspondingly poorer in augite 

 and ilmenite. These differences cannot be due to fractional 

 crystallisation, because while biotite was one of the earlier 

 minerals to crystallise, felspar was one of the later, and 

 these two minerals are notably coincident in this zone. The 

 junctions between this zone and the zones of pallio-essexite 

 and normal essexite above and below are not absolutely 

 sharp, although the change from one to the other is fairly 

 sudden. The distribution of the minerals is not uniform 

 throughout this felspathic zone, for while everywhere 

 higher in felspar than the normal essexite, that part 

 adjacent to the pallio-essexite is more basic than that lower 

 down (see table on page 514 in our previous paper). This 

 upper portion is also finer grained than the lower portion. 



Two possible causes suggest themselves in explanation 

 of these features, (a) gravitational sinking of the heavier 

 minerals during the earlier stages of crystallisation, (b) a 

 partial liquation of the magma before crystallisation had 

 begun, i.e., a separation into two layers, the one more salic 

 and of lower specific gravity above, the other more femic 

 and heavier below. While admitting that liquation may 

 have been a contributing cause, we are of opinion that 

 gravitational sinking of the heavier minerals was the main 

 cause. The fact that the upper part of this zone is more 

 basic than the lower may be explained as being due to its 

 more rapid solidification, as shown by its finer grainsize, 

 having allowed less time for gravitational sinking to 

 operate. 



I— July 3, 1912. 



