374 Transactions. — Geology. 



In one of the felspar crystals a remarkable structure was 

 observed. What appeared to be an ordinary narrow band 

 was crossed at small intervals by black marks, so that the 

 whole appearance was like the rungs of a ladder. On ex- 

 amining it with a higher power no difference was observed in 

 the structure. The bars were black throughout the revolution 

 of the stage, whether the nicols were crossed or not, so that it 

 was probably some alteration product produced in a rather 

 curious way. (See PI. XLVIL, fig. i.) 



2. Olivine. — The olivine occurs in rounded crystals, which 

 are crossed by irregular cracks. Some of these are so much 

 corroded that they show no trace of their original form (see 

 PL XLVIL, figs, hi., iv., and v.) ; but in a few cases faces were 

 seen remaining, as in fig. iv., in which the extinction bisected 

 the angle between the straight edges. The alteration which 

 marks nearly all the olivine present is most strongly observed 

 near the periphery and along the cracks. The alteration 

 product is limonite. It is shown in the figures by the dark 

 shading. Sometimes unaltered fragments are seen, and they 

 show the brilliant polarisation colours and rough surface of 

 olivine. Considering the amount of olivine which has been 

 present, the percentage of magnesia (2-75) is rather low, but 

 nearly all the magnesia has been taken away by weathering 

 action, and there is left an aggregate of limonite. 



3. Augite. — The amount of augite in this rock is very 

 small compared with the amount of olivine, and the last- 

 mentioned mineral is the most prominent ferro-magnesian 

 mineral. Only a few crystals of augite were observed in the 

 sections made, and these were a pale greenish-brown by ordi- 

 nary light. They showed no pleochroism, but moderately 

 brilliant colours under polarised light. They were distin- 

 guished from the unaltered olivine by their smooth surface 

 and by the absence of irregular cracks. Some showed a 

 banding, evidently due to polysynthetic twinning (fig. ii.). 



As this is parallel to aP , it gave a means of determina- 

 tion by extinction when an angle of 39° was obtained. In 

 fig. ii. the longer lines represent bandings which also cor- 

 responded to cleavage-cracks, while the shorter irregular 

 cracks were parallel to the prism. 



4. Magnetite. — The presence of this mineral was shown 

 by examination with reflected light, when numerous black 

 grains appeared, with the blue metallic lustre of magnetite. 

 The presence of this mineral will account for the ferric oxide, 

 which was shown to be present by means of chemical analysis. 

 No other porphyritic minerals were noticed. 



