DIFFERENTIATION PHENOMENA OF THE PROSPECT INTRUSION. 



127 



Most of these layers are shown in Fig. 1. The felspathic 

 phase of the essexite (layer B) contains the two large 

 segregation veins as shown ; these will be dealt with later 

 and will not be further discussed in this section. 



The mineral composition of these different layers is as 



follows : — 



Table I. 





Pallio-essexite. 



Essexite, 



Felspathic phase 



(excluding the 



Segregation 



Veins) . 



Essexite, 

 normal phase. 



Felspar (Plagioclase) 



37-39% 



45-56% 



38% 



Augite 



30-33 



20-34 



38 



Olivine 



14-21 



4-8 



5 



Ilmenite 



4-7 



3-14 



16 



Biotite 



0-11 



2-7 



1 



Apatite 



O'l-l'O 



0*7-2*0 



0'4 



Orthoclase 



1 



0*15-5 



2 



A. Origin of the Pallio-essexite. — This forms a selvage 

 to the intrusion and has a thickness of from three to four 

 metres. It will be seen from the above table that the 

 border facies of pallio-essexite is (a) richer in olivine and 

 biotite, (b) poorer in augite and ilmenite than the normal 

 essexite ; further its felspars, although about equal in 

 quantity are somewhat more acid. In addition the pallio- 

 essexite is aphanitic in texture, whereas the essexite is 

 phanerocrystalline. 



The larger proportion of olivine, and perhaps also of the 

 biotite, appears to be the result of fractional crystallisation 

 at an early stage of solidification. The outer zone of the 

 intrusion in contact with the cold shales, would, owing to 

 rapid conduction, soon have a somewhat lower temperature 

 than the main mass of the intrusion. Crystals of olivine 

 would begin to develop in this zone at a time when the 

 main mass of magma was still too hot for this mineral to 

 crystallise. For olivine to develop in excess of the amount 



