Notes on the Doleri'tic Intrusions of East Fife. 445 



digression may here be permissible in reference to tbe determina- 

 tion of these acid plagioclases, and a method of examination 

 indicated which the writer has found to yield valuable information. 

 The optical dispersion of Canada balsam is greater than felspar, 

 and by passing from the red to the blue end of the spectrum a fair 

 range in refractive index can be obtained from the same medium. 

 That is to say, supposing the field of the microscope to be illumined 

 by red light the index of the balsam will be relatively low, but by 

 a change in the character of the light the index can be readily 

 increased. If therefore while one of these felspars is under observa- 

 tion the wave-length of the illuminant be progressively shortened, 

 it is not difficult to see for what colour there is an agreement in the 

 matter of refractive index. ^ Tbe disjiersion of the balsam and of 

 the felspars being fairly well known, it is possible by such m.eans 

 to get a rapid and accurate indication of the nature of the latter. 

 One of the first results obtained from an application of the above 

 principle was a discernment of the fact that a lime-soda felspar 

 may have adjacent lamellae of very different composition, i.e. 

 one lamella may perhaps be labradorite, those contiguous oligoclase. 

 Such assemblages do not yield symmetrical extinctions in the zone 

 normal to (010), and ordinarily would probably be neglected by 

 the petrographer. 



Reverting to the Gathercauld dolerite, it may be noted that the 

 felspars enclosed by the pyroxene are labradorite, whereas the 

 later-formed and interstitial felspar is acid plagioclase. It is 

 interesting to observe the manner in which the latter frequently 

 wraps round the terminations of the earlier product, and it is not 

 uncommon to find a series of laths of more highly refracting 

 labradorite enclosed in a felspathic medium of distinctly lower 

 index. 



The pyroxene is reddish or brownish in sections that show the 

 characteristic prismatic cleavage, but in sections parallel to the 

 crystal axis c is faintly purple and visibly pleochroic. The axis 

 emerging upon the base, or more accurately between the bisectrix Z 

 and c, is markedly dispersed (r > v) in the symmetry plane, but this 

 dis])ersion cannot be observed so favourably in the case of the other 

 axis owing to an increment in the angle Z — c with diminishing wave- 

 length, it thus happens that in the same rock section axes may 

 be found that are nearly " achromatic," others being strongly 

 dispersed. A noteworthy point with reference to the pyroxene 

 is that whereas it is ophitic towards the earlier felspar, it is sharply 

 idiomorphic towards the later interstitial felspar. 



Olivine is abundant in the rock of Gathercauld, frequently 

 enclosed by pyroxene, or lying sim,ply in a meshwork of felspar 

 laths, but preserving its contour and rarely penetrated by the 



^ It is difficult with a spectroscopic illuminator to get a sufficient intensity 

 of light : isolation of the lines of the mercury arc has been found to give useful 

 results. 



