130 



H. S. JEVONS, H. I. JENSEN AND C. A. SUSSMILCH. 



O. The Normal Phase of Essexite. — This appears to 

 constitute the great bulk of the intrusion, its total thick- 

 ness is unknown, and the maximum thickness exposed is 

 about sixty-five feet (in the Reservoir Quarry). As com- 

 pared with the original undifferentiated magma it has been 

 impoverished by the concentration of olivine in the pallio- 

 essexite, by the relative concentration of felspars and 

 biotite in the felspathic zone, and also by the concentration 

 of alkaline minerals (albite and egerine) in the segregation 

 veins. To what extent it differs from the original magma 

 as a result of these impoverishments it is, in the absence 

 of any positive knowledge as to its thickness, impossible to 

 say. It is probable, however, as stated above, that the 

 rapidly cooled pallio-essexite, after deducting the excess of 

 minerals resulting from fractional crystallisation, most 

 nearly represents the original composition of the magma. 

 2. The Segregation Veins. 



The nature and distribution of the segregation veins has 

 been fully described in our previous paper (p. 526). The 

 two large veins therein described, occur in the felspathic 

 zone of the essexite, and one of them is shown in figure 1. 



Pallio- Essexite. 



Essexite. 



:*j Ujjpci'Sc^regatioT) Vein| 

 £^ Essexite. 



Fig. 1 . — Sketch of part of the face of the Reservoir (old) Quarry. 



