GEOLOGY AND PETROGRAPHY OF THE PROSPECT INTRUSION. 515 



seem to be important, as our observations all tend to show 

 that essentially the same changes in composition take place 

 in a vertical direction in all parts of the mass which are 

 exposed. Whilst in the figures quoted in the table for 

 felspar orthoclase has been included, it has seemed desir- 

 able to repeat the orthoclase separately for those specimens 

 which have been analysed, and in which its quantity can 

 therefore be determined with some certainty. 



Directing attention first to the rocks of the rapidly cooled 

 envelope (columns I and II), we notice that they are similar 

 in composition except for the proportion of biotite, which 

 in specimen L is practically absent, but in specimen B is 

 present to the extent of nearly 12 per cent. Examination 

 of other specimens shows that biotite is usually abundant 

 in the pallio-essexite ; and its absence in one of these 

 specimens is an excellent instance of the irregularity of its 

 distribution, which is seen to be very marked when many 

 specimens from all parts of the mass are compared. On 

 the whole, biotite tends to be most abundant in a zone 

 within 5 m. of the junction with the shales, its percentage 

 there being generally from 6 to 12, whilst in the interior 

 of the mass its proportion usualy lies between one and five 

 per cent. Yet from any part of the mass a specimen may 

 chance to be taken which contains very little biotite. In 

 other respects specimens of the pallio-essexite all agree; 

 olivine is very abundant, the iron ores moderately so, whilst 

 augite and felspar usually appear in about the same pro- 

 portion, as in specimens L and B (columns I and II). 



On passing downwards into the mass a striking change 

 in composition is apparent. Felspar continually increases 

 in proportion, whilst olivine and biotite fall off in a very 

 marked degree. It is true that 5 cm. from the junction 

 biotite is almost absent ; but this may fairly be regarded 

 as irregular, being probably due to reaction with quartz 



