466 PROP. W. S. BOULTON ON A MONCHIQT7ITE INTRUSION [NOV. I9II, 



Of these corroded olivine-phenocrysts represent portions of the 

 plutonic xenoliths or 'nodules' which contain abundant olivine 

 (see p. 471). 



The ground-mass enclosing these large phenocrysts contains 

 smaller porplryritic crystals of a second generation, including augite 

 and olivine, with occasional biotite, together with chromite and 

 rounded quartz-crystals, all set in a fine matrix which, under a 

 high power, resolves itself into analcite, embedded in which are 

 minute prisms of augite, plentiful octahedra of iron-ore, scales of 

 biotite and minute needles of apatite ; while secondary products, 

 such as carbonates, idiomorphic analcite and other zeolites, chlorite, 

 haematite, and limonite are locally abundant, especially in the ocelli 

 and steam-cavities. 



The porphyritic augites of the second generation, ranging 

 in length from 1 mm. down to the minute prisms of the ground- 

 mass, are well-formed, idiomorphic or sub-idiomorphic crystals, with 

 their angles perfectly sharp, and presenting the usual octagonal 

 and prismatic sections. The colour is purplish brown, with slight 

 pleochroism. Twinning is rare. Purplish-brown crystals can be seen 

 with a nucleus of nearly colourless and partly-decomposed augite, 

 that doubtless represents the last remnant of the resorbed larger 

 phenocryst, on which the later augite has crystallized out in optical 

 continuity. It sometimes surrounds altered olivine, and very often 

 encloses crystals of biotite. Crystals occur with the centre pale in 

 colour and decomposed, surrounded by brown augite enclosing in a 

 zonal manner numerous flakes of biotite. 



Olivine of the second generation occurs in small idio- 

 morphic crystals (averaging 03 mm. in length), showing the 

 typical bipyramidal forms and cleavage-cracks, but now entirely 

 represented by pseudomorphs of serpentine, iron-oxide, calcite, and 

 colourless zeolites. 



The chromite or picotite varies in colour from deep reddish- 

 brown to brownish-yellow. It is usually in rounded crystals where 

 embedded in the matrix, their outlines being often emphasized by 

 broad, irregular, black borders of iron-ore. 



The biotite closely resembles the earlier phenocrysts. 



The quartz -grains are perfectly clear, except for the usual 

 minute, linear inclusions. They invariably show rounded outlines, 

 and are often considerably embayed by the ground-mass, with 

 a yellow or dark corrosion-border of fine granular substance sur- 

 rounding them (fig. 3 D, p. 467). In these characters they resemble 

 the quartz-inclusions or xenocrysts in other well-known basic 

 lamprophyres that have been described. 1 



The ground-mass under alow power is typically brown or 

 dark grey, plentifully dusted over with minute magnetite-grains. 

 But a high power reveals a very interesting structure, which throws 

 considerable light on the composition and affinities of the rock. 



1 See A. Harker, Geol. Mag. dec. iii, vol. ix (1892) pp. 199-206 & 485-88. 



