Speight. — On an Occurrence of Olivine-andesite. 373 



/?. By Examination of Cleavage-flakes. — Since the crystals 

 of plagioclase were not large, the only way to obtain cleavage- 

 flakes was by breaking the rock up and looking over the 

 crushed fragments. The following which were obtained 

 furnish further evidence. They were examined by conver- 

 gent light and also with ordinary polarised light, to determine 

 the extinction-angles : — 



(1.) From the brachypinacoid. In convergent light gave 

 an axis just out of the field, with a revolving axial shadow. 

 Angle of extinction —9°. 



(2.) Also from the brachypinacoid. Gave results similar 

 to (1), but the angle of extinction was —4°. 



(3.) From the basal pinacoid. Gave an axial shadow and 

 optic axis just out of the field. Angle of extinction was —5°. 



In this cleavage - flake the twin lamination appeared as 

 alternate bands, but one set were so fine that they became 

 mere striae, while the other were broad. This gave an inter- 

 ference figure produced from the broad bands, and also afforded 

 a definite line for determination of the extinction-angle. In 

 the other cases this line was not so satisfactory, as straight 

 edges, which were taken to be lines of cleavage, were 

 used. 



(4.) From the brachypinacoid. This gave an interference 

 figure in which an oblique bisectrix was clearly visible. The 

 shape of the figure showed it to be ohgoclase. In parallel 

 polarised light the angle of extinction was 18°. 



The felspar has thus been almost conclusively shown to be 

 labradorite, with the occasional occurrence of oligoclase. It 

 is marked with well-defined polysynthetic twinning. Some of 

 the bands are extremely delicate, while in some cases binary 

 twins were noticed. 



A well-defined zonal structure appeared in many crystals. 

 This may be due to successive accretion under different condi- 

 tions of temperature and pressure, or it may be produced by 

 strain. There is positive evidence that the rock has suffered 

 strain. A large number of felspar crystals show undulose 

 extinction, and this has been shown to be common to the 

 ground-mass as well. In one section a small vein of rock 

 occurred about ^gin. long. It was composed of crushed frag- 

 ments of labradorite, and some of these showed twinning, but 

 others only a delicate striation, so fine as hardly to be noticed. 

 This may be evidence that polysynthetic twinning is due to 

 pressure, as we are led to believe from experiment. This vein 

 was produced during the process of cooling, wlien the rock had 

 almost consolidated. In another case a crystal exhibited a 

 fine striation perpendicular to the general banding. This was 

 perhaps a twin on the pericline type, but the marking was 

 very faint. 



