GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 517 



position generally closely similar to that of specimen P 

 (column IV), though we have not determined quantatively 

 any specimen from this zone. Above, between, and below 

 the two massive aplitic veins of the Reservoir Quarry, the 

 rock is decidedly felspathic in character, probably similar 

 in composition to the Survey specimen (column V) though 

 with less olivine and biotite, but after passing little more 

 than a metre below the lowest aplitic vein we find that the 

 rock has very rapidly become much less felspathic again, 

 as shown by the composition of specimen I (column VII) 

 which was taken fully 18 metres below the junction. This 

 specimen is the richest of any in augite and iron ores; it 

 has about the same proportion of felspar as the pallio- 

 ossexite, though rather more of the anorthite molecule, a 

 moderate percentage of olivine, and very little biotite. The 

 main rock underlying the lowest aplitic vein seems every- 

 where to be comparatively uniform in composition. It is 

 only at the north-west end of the Reservoir Quarry that 

 the rock is exposed to any depth below the aplitic veins : 

 here the latter turn upwards parallel to the upper contact 

 surface (see Plate XXXV) because the section line of the 

 quarry runs into the raised outer rim of the mass referred 

 to above; and, thus, though we get to no absolutely deeper 

 level, we do, at this end of the quarry, get specimens from 

 a greater depth below the top of the igneous mass than is 

 possible anywhere else. Unfortunately it is also here that 

 the rock is most decomposed, because denudation has 

 removed the protecting cover of pallio-essexite; but so far 

 as we have been able to observe the rock at this locality, 

 it does not appear to present any of the rapid changes in 

 composition which are to be found in the overlying 20 

 metres (65 feet) or so of rock. Hence we may assume 

 specimen I to be typical of the main rock everywhere 

 throughout the mass at depths lying between 20 and 40 

 metres (65 and 130 feet) below the upper contact surface. 



