Vol. 67.] IX THE OLD RED SANDSTONE OF MONMOUTHSHIRE. 465 



The mineral is typically black, sometimes greenish, with a dull 

 pitchy lustre and conchoidal fracture. The crystals that have 

 suffered corrosion to the greatest extent seem to be* those which 

 exhibit a specially dull lustre, while smaller ones have the more 

 usual augitic lustre. 



It is much cracked, with limonite, chlorite, and secondary car- 

 bonate along the cracks. This strong tendency to crack makes it 

 practically impossible to break off specimens of the rock with very 

 big augite-phenocrysts entire. The crystals that have been much 

 corroded break away from the enveloping rock very readily, 

 evidently because the zone of fused and recrystallized augite (see 

 below) that surrounds the inner core makes a weak contact with 

 the matrix. 



Seen in thin section the mineral is pale green, nearly colour- 

 less, occasionally a very pale pinkish-brown. In thicker sections 

 the colour is olive-green with slight pleochroism, the colours varying 

 from pale olive-green to brownish green. Besides showing the usual 

 prismatic partings, the mineral is much cracked, a yellowish-green 

 substance filling most of the cracks. Small rounded crystals of 

 altered olivine sometimes occur pcecilitically ; but, apart from these, 

 and rods and flakes of haematite, the crystals show remarkably few 

 inclusions. Xo cases of twinning were met with. 



It is probably a chrome-diopside, a variety of augite common 

 in rocks of this class. Generally, the borders show an outer zone 

 of resorbed augite, with a faint granular polarization ; and, in many 

 instances, this border has been supplemented by an outer zone of 

 fresh augite that crystallized out from the magma immediately after 

 the corrosion of the phenocr}*st. This outermost zone is purplish 

 brown, appreciably pleochroic, and quite distinct in colour from 

 the original crystal. It is generally made up of minute stout 

 prisms, oriented with the principal axis of the phenocryst, so that 

 they all extinguish together, and uniformly with the original inner 

 core. (See PI. XXXVII, fig. 1.) 



Biotite. — Phenoerysts of biotite measuring as much as 2 inches 

 across are plentiful in some parts of the rock, and they sometimes 

 exhibit a parallel arrangement. They are in six-sided plates, but 

 usually the angles have been worn off b}* corrosion, so that they are 

 circular or oval. 



By transmitted light they show a deep colour, with strong 

 pleochroism, the colour varying from yellow to reddish brown. 

 Examples can be noted where the crystal is deeply embayed by the 

 matrix, or again, where the crystal has been almost entirely eaten 

 up by the magma. Many of the crystals are bent and frayed out 

 at the edges, indicating violent movement and friction since their 

 formation. 



Olivine. — Idiomorphic olivine-crystals, now entirely replaced 

 by serpentine, carbonates, etc., are corroded in much the same 

 way as the augite and biotite. It is probable, however, that some 



