GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 519 



Reference to the foregoing table shows that the pallio- 

 essexite contains from 5| to 1\ per cent, of iron ores (see 

 columns I, II, and III) the quantity diminishing somewhat 

 away from the contact. Going downwards, the iron ores 

 then increase considerably, fall off again in the highly 

 felspathic varieties, (Survey specimen and aplite), and then 

 increase to their maximum in the main rock below the 

 segregation veins. This rather complex arrangement might 

 perhaps be accounted for by supposing that the average 

 proportion of iron ore, which in the pallio-essexite is about 

 7 per cent., would normally have increased downwards 

 rather rapidly to the 16 per cent, found in the main rock 

 below; but that continuous increase downwards was pre- 

 vented by the concentration of the felspars in the level 

 between 5 and 17 metres below the junction, the dilution 

 by felspar of course reducing the percentage of all the 

 other minerals. If augite be taken as a standard of refer- 

 ence, because it diminishes with increase of the felspar, we 

 find that the ratio of total iron ores to augite is variable? 

 but that in passing from specimen P to the Survey specimen, 

 when the felspar changes from 50 to 56i per cent., the ratio 

 of ores to augite is nearly constant, thus perhaps pointing 

 to the dilution theory as acting between these two 

 specimens. 



Changes in the proportion of the iron ores are not found 

 only in a vertical direction, or from the exterior of the 

 mass inwards, but also in an apparently irregular manner. 

 The best example of this is to be found in the Reservoir 

 Quarry, a little to the south-east of the middle of the face. 

 In the immediate neighbourhood of the great segregation 

 veins, and particularly in their pegmatitic portions, the 

 ilmenite crystals attain great. size and generally lie nearly 

 horizontally. They form a striking feature in the hand- 

 specimens, appearing as very thin hexagonal tables or 



