BY W. G. WOOLNOUGH. 511 



refractive indices proves that the felspars are complex in structure. 

 Many of the crystals exhibit a large kernel with a refractive 

 index about the same as that of quartz (sometimes slightly higher, 

 sometimes lower), but always higher than that of Canada balsam. 

 These kernels are surrounded by a broad, very sharply defined 

 ring of felspar, comparatively free from the cracks which are so 

 abundant in the kernel, and with a refractive index lower than 

 that of either quartz or Canada balsam. In most cases there are 

 only two such sharpl}^ defined zones, but in a few instances there 

 are three or several rings. 



In polarised light most of the sections exhibit albite lamellae, 

 with or without Carlsbad and pericline twinning. Some sections, 

 proved not to be orthoclase by their refractive index and optical 

 properties, show only "the Carlsbad twinning without any trace 

 of lamination after the albite law. In addition to the compara- 

 tively simple twinning, there is, in many instances, an excessively 

 fine polysyuthetic twinning sensibl}^ perpendicular to the albite 

 lamellse, and usually affecting only a portion of the section. It 

 is possible that this may represent the twinning after the pericline 

 law, but as the latter form occurs well developed in other sections, 

 this particularly fine lamination is very puzzling. 



In spite of the comjDarative freshness of the felspar, it is 

 difficult to find a section giving satisfactory measurements. The 

 crushing of the rock as a whole has given rise to bending and 

 faulting of the lamellae, undulose extinction, and aggregate 

 polarisation. Sections parallel to (010) are on the whole more 

 satisfactory than those perpendicular to that face. The extinc- 

 tion of the outside zone varies from within outwards from- 6*^ 

 to-2^ that of the kernel from -28^^ to -16^'. Taken in con- 

 nection with the refractive index observations, these figures 

 indicate a felspar varying between labradorite and andesine for 

 the kernel, and zones of oligoclase of varying composition for the 

 peripheral portions. 



The sudden change in the composition of the felspar between 

 the kernel and the outer coating is suggestive of a marked change 

 in conditions of crystallisation after the formation of the inner 



