The Geology of Coimaidai. 73 



crystals and blebs of olivine. The rock is microscopically bolo- 

 crystalline, approaching to a pilotaxitic structure. The constituent 

 minerals are felspar, olivine, augite and magnetite. The felspar 

 is in lath-like forms, having their longer axes arranged in more or 

 less parallel lines ; there is some trace of flow structure round 

 the larger phenocrysts of olivine and augite. Olivine is very 

 abundant in the slide, appearing both as allotriomorphic crystals 

 and as grains. The crystals are large and almost entirely free from 

 decomposition. Augite is less abundant than olivine ; it occurs 

 in allotriomorphic crystals, rounded masses and clustei's. Magnetite 

 is seen in large quantities, as minute specks often forming a 

 wreath or border round the outer edges of the phenocrysts, as 

 small grains, and in large masses presenting certain peculiar 

 features. One of these masses is not unlike a semi-circle in 

 shape, the side corresponding to the diameter of the semi-circle is 

 between '02 and "03 of an inch long, and is a fairly straight line, 

 while the curved edge is deeply fretted and has a lattice-like 

 structure, the minute areas bounded by the thin films of magnetite 

 contain anisotropic matter, probably a devitrified glass. 



Slide No. 3 is from a specimen taken from a small outcrop in 

 Langmuir's paddock, about two miles W.N.W. of Coimaidai. 

 This outcrop is the only one found by us west of the Goodman's 

 Creek. Macroscopically it is very similar in appearance to 

 the rock last described, porphyritic crystals of augite, decom- 

 posed olivine and a reddish-brown mica may be seen by the naked 

 eye. The base may be classed as hypociystalline, being made up 

 mainly of grains of magnetite and augite, with residual glass. The 

 minerals present are augite, olivine and magnetite. Felspar is 

 almost entirely, if not quite, absent. Augite is abundant in 

 well preserved idiomorphic crystals, often showing zonal structure. 

 A peculiar feature is exhibited by some of the augite phenocrysts. 

 When the stage of the microscope is rotated between crossed 

 nicols, the crystal is seen to be composite in character, there being 

 a central area and a peripheral area ; the boundaiy of the central 

 area does not conform to that of the crystal, and the two areas 

 do not obscure simultaneously. The cleavage lines of the inner 

 and outer areas are continuous. The appearance presented is 

 quite dissimilar from tliat of normal zonal structure, and points 

 to considerable discontinuity in the growth of the crystal. Other 



