552 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



mate the coloured constituents of a rock, owing to the 

 fact that where crystals of the latter overlap a colourless 

 constituent the whole width of the overlap, and not only 

 half, is reckoned as part of the coloured constituent. The 

 latter source of error is accentuated by the fact that the 

 slides used are rather thick. The difference of the biotite 

 is assumed from inspection of the slides, as already stated. 



Appendix III. 



Note. — The following paragraph should have appeared at the 

 end of Part III, on page 540, but was accidently omitted. 



The question now arises as to the naming of the rocks 

 occurring in the segregation veins. Although they differ, 

 particularly in the case of the aplite, in their mineral and 

 chemical composition from the essexite, they are so essen- 

 tially a part of the essexite intrusion that any names given 

 should show that connection ; we therefore propose the 

 names essexo-pegmatite and essexo-aplite for these two 

 rock types. The names aplite and pegmatite have usually 

 been restricted in their use to the segregation veins occur- 

 ring in granite ; there is, however, no reason why they 

 should not be applied to similar segregation veins occurring 

 in any igneous rocks, the names being modified as above to 

 indicate the parent rock to which they belong. 



Explanation of Plates. 



Figs. 1, 2, 3. Diagrammatic sections of the Prospect intrusion. 



Fig. 4. Rough sketch of that part of the upper segregation vein 

 shown at H on Plate XXXV (transfer). 



Plate XXX IV. 

 Geological map of the Prospect Intrusion, New South Wales, 

 also sketch section on line AB. 



