346 H. J. Lowe — Rock Differentiation. 



long dull redclish-brown crystals, some attaining a length of 15 mm. 

 Fresher rock in hand-specimen is of greyish-blue colour. A micro- 

 section displays hornblende and felspar as the chief constituents of 

 the rock, with iron-ores and apatite as accessories. The secondary 

 minerals comprise chlorite, calcite, mica, and leucoxene. The horn- 

 blende is of a rich brown colour, and is in well-developed crystals of 

 A-arying sizes, showing crystal faces and cleavages very perfectly. 

 Constitutionally, however, the hornblende has much suffered by decay. 

 The crystals are frequently traced with chlorite, and are met with in 

 all stages of chlorite transformation, even to a complete paramorph of 

 the secondary mineral, with the original crystal outlines remaining 

 perfect. The pleochroism of the hornblende is very pronounced from 

 a very pale brown to a rich red brown, with birefringence colours 

 ranging from bright yellow, golden brown, to a greenish-red lustre in 

 moderately thick sections. Twinning is seen occasionally in the better 

 preserved crystals. 



The felspar presents anomalistic relations. Some appears in 

 irregular plates with very indefinite boundaries ; part in lath shapes, 

 in sheaf or fan arrangement, with still smaller forms in curved and 

 feathery microliths. The lath-shaped felspars are frequently attached 

 in an irregular way to the phenocrysts, giving them an appearance of 

 frayed out edges, not always in crystalline continuity. They some- 

 times, however, occur in sheaves or single laths in spaces filled with 

 a groundmass material, which is nearly isotropic. This interstitial 

 matter is mainly resolved by higher power into minutelj' crystalline 

 material, probably of a felspathic nature, amidst which the curved 

 and feathery shapes are intermediate in size to the laths. Here also 

 occur occasional small imperfect hornblendes, partially decomposed 

 coloured mica flakes, as well as chlorite and calcite granules. The 

 larger felspar plates which frequently enclose the hornblende are 

 much altered, the secondarj' product therefrom appearing as brilliantly 

 polarizing flecks of colourless mica. These being formed along the 

 cleavage planes, their consequent parallelism permits series to 

 extinguish together. Occasionally a cleavage trace may be seen in 

 these larger felspars, indicating simple twinning, but the evidence of 

 poly synthetic albite twinning is too rare and faint to deem it a habit 

 in the development of these phenocrysts. Marginal differences, 

 noticeable in the partial extinction, declare zonal variation in original 

 constitution. The extinction obtainable in the larger felspars is most 

 often within very small angular distance of available lines of 

 measurement, occasionally identical therewith ; while in the laths 

 extinction takes place almost invariably with the cross line parallel 

 to the long axis of the crystal. These indications correspond to those 

 of a plagioclase of varying constitution, related to the acid end of the 

 series, oligoclase and andesine, the lath crystals being probably of 

 a more acid character than the larger felspars. 



Of secondary minerals, chlorite is the most abundant in its common 

 form of strings of fan-like concretions, and calcite is common in both 

 compact and granular condition. Pyrites is sometimes present, though 

 rare, and ilmenite is a sparingly common accessory, often assuming 

 the semi-translucent condition, indicating its change to leucoxene. 



