130 



amount of orthoclase. Strongly pleochroic brown-green horn- 

 blende is the predominant femic silicate, uralite edged with 

 very small epidote grains being present in but small amount. 

 A fair amount of quartz is present. Titanium is present in 

 ilmenite, slightly leucoxenized, and sphene, both of which 

 are plentiful ; while apatite is present in grains and prisms 

 up to ^ mm. in length. 



No. 590 differs from the above in the abundance of the 

 uralite. The felspar is an acid plagioclase, and is the pre- 

 dominant mineral. Pericline twinning is often well devel- 

 oped. It is very full of liquid inclusions, which run in 

 bands through the crystal irregularly, or is concentrated in 

 rows parallel to the twinning plane (010) and the basal 

 cleavage. The uralite is a pale-grassy-green, with noticeable 

 pleochroism. The M-ihole surface of uralite is dotted with 

 tiny epidote grains, giving it a very roughened appearance, 

 while a strong border of epidote has formed all round the 

 uralites and apatites. A little limonite sometimes occurs in 

 the uralite. Brown-green hornblende is subordinate to the 

 uralite, and is quite fresh ; while ilmenite, apatite, and quartz 

 are the accessories in order of relative abundance. 



No. 594 is remarkable for the predominance of the femic 

 constituents. Hornblende is present in greatest abundance in 

 idiomorphic to irregular grains, often twinned. Diopside 

 was present in almost equal amount. It has become to a 

 large extent changed to a pale-green uralite, becoming along 

 the edges of the grains blue-green and strongly pleochroic. 

 The uralite has been somewhat chloritized, and the fibres of 

 chlorite do not remain parallel to the vertical axis of the 

 diopside crystals. Of the felspar present, in amount nearly 

 equal to the coloured silicates, plagioclase is distinctly 

 the predominant. Its refractive index is slightly above that 

 of Canada balsam, and is a basic oligoclase. It shows some 

 pericline twinning, slightly developed. The orthoclase is 

 quite untwinned. Both felspars are almost free from kaolin- 

 ization, though a little is developed along cleavage planes. 

 The characteristic accessories of the rocks of the Houghton 

 magma — ilmenite and apatite — are typically developed. Epi- 

 dote occurs in small amount. (See plate iv., fig. 7.) Name. 

 — Diorite. 



vii. Boulder frniv Camhrian Till, Jamestown. 



Macroicopical. — Aphanitic pale-green translucent rock 

 with phenocryst of felspar up to 5 mm. in diameter and cubes 

 1 mm. in diameter, of black-lustrous, weakly magnetitic, 

 slightly titaniferous iron-ore. Microscopical (see plate iv., 

 fig. 8). — Porphyritic, with phenocrysts of oligoclase-albite, 

 fairly idiomoiphic, and showing pericline twinning. It is 



