MacLeod. — Ovi a Hornblende Trachyte. 491 



In this analysis the high percentage of SiO.,, which is 

 a uoted feature in many of our rocks, is most likely due to in,- 

 liltrated silica, patches of which are clearly discernible under 

 the microscope. 



Under the microscope the constituent minerals are seen to 

 be feldspars and hornblendes set in a base, and following the 

 natural order of consolidation. The hornblendes vary greatly 

 in size, some being quite microscopic, whilst others are over 

 ^ in. in length. T-shaped twins are visible in some sections. 

 Their colour by ordinary light is brown, with a faint tinge of 

 green, and on inserting the polariser the pleochroisra is very 

 marked, and is as follows : — 



y — Dense green-brown. 

 (3 = Yellow-brown. 

 a = Honey-yellow. 



The cleavage is well defined, especially the prismatic, which 

 is well shown both in longitudinal -and cross sections. The 

 polarisation colours vary from yellow-brown to dark green- 

 brown, aiad the extinction angle C : c varies from about 14° to 

 16°. The hornblendes show a well-marked dark border-line, 

 due to corrosion in the molten magma previous to consolida- 

 tion. 



The feldspars vary in size from quite microscopic plates 

 and laths up to 5 mm. to 6 mm. long. By ordinary light they 

 appear as colourless plates, with here and there a few colour- 

 less laths. Under polarised light they appear much twinned, 

 aad exhibit zonary banding, and the extinction angles varying 

 from about 4° to 10° point to their being oligoclase, though 

 the analysis showing some 2 per cent. KgO would point to the 

 probability of some of the feldspar being anorthoclase. 



The chief accessory minerals are magnetite, occurring in 

 the hornblendes, and infiltrations of chalcedonic silica in 

 cavities in the base. Negative crystals and gas inclusions are 

 very numerous in the feldspars, whilst inclusions of the latter 

 mineral are found in the large hornblendes. 



The base consists chiefly of a crypto-crystalline aggregate 

 of feldspathic matter, with here and there small granules of 

 feldspar scattered through it. 



Appended (Plate XLVIII.) are two water-colour sketches 

 of sections of this rock — No. 1, under ordinary light; No. 2, 

 under crossed nicols. i — inclusions of silica ; / = feldspars ; 

 h = hornblendes. 



