460 PEOF. J. W. JUDD AND G. A. J. COLE ON THE 



the mass of brown glass. These crystallites appear, with the 

 highest power of the microscope, as excessively minute globular 

 bodies (globulites), which in some parts of the mass are crowded 

 together into cumuli tes, leaving other parts comparatively clear and 

 transparent (PL XTY. fig. 1). Sometimes an approach to a linear 

 arrangement can be detected in these globulites. The cloudy masses 

 exhibit in some cases the parallel grouping characteristic of the 

 banded or fluidal structure. 



In the basalt-glass of Lamlash this linear arrangement of the 

 globulites is much more strikingly exhibited than in that of the 

 Beal in Skye. Here we sometimes find the resemblance to strings 

 of pearls which has led to such objects being called " mar- 

 garites." Some of the globulites are seen to be transparent bodies ; 

 and this is beautifully shown by the Lamlash rock (PI. XIV. fig. 2). 



The basalt-glass of Some, Screpidale, and Gribun shows a further 

 development of the embryo crystals. The fine globulites are seen 

 to be collected into skeleton crystals quite similar in form to those 

 found in so many slags, but usually of much smaller dimensions. 



In addition to these, we find in the Screpidale rock abundant 

 transparent colourless rod-like bodies (belonites), and in that of 

 Gribun curious examples of forms intermediate between skeleton 

 crystals and spherulites, which occur in the Hawaiian lavas, and 

 have received from Krukenberg* the name of " chiasmoliths." 



As to the nature of the crystals which are thus found in an 

 embryonic and partially developed condition in these glasses, much 

 discussion has taken place. The minute opaque forms, which seem 

 to have the symmetry of the cubic system, can scarcely be other 

 than magnetite ; but the larger and more transparent forms, such 

 as are seen, for example, in the chiasmoliths, appear to be augite, 

 while the belonites are probably for the most part felspar. The 

 margarites in the Lamlash rock may not improbably be felspar 

 crystals in their earliest stage of development. 



Spherulites occur in the rock of the Beal. and also in the some- 

 what similar rock (augite-andesite-glass) of Beinn Shiant, Ardna- 

 murchan. 



In some cases, as in that of the Beal near Portree, the change 

 from an ordinary basalt which occupies the centre of the dyke to 

 the glass at its sides can be traced step by step. The porphyritic 

 or first-formed crystals are alike in both the basalt and the glass, 

 and consist mainly of olivine and felspar, the latter much rounded 

 and corroded by the action of the glass upon it. In the glass at 

 the extreme edge of the dyke the globulitic dust, occasionally col- 

 lected into cumulites, fills the whole mass ; nearer the centre this 

 glass has the globulites united into skeleton crystals of magnetite. 

 The basalt near the glass contains a very large quantity of unin- 

 dividualized glass, in which transparent microliths of felspar and 

 augite can be made out under a high power. In the basalt of the 

 centre of the dyke the felspar, augite, and magnetite are well 



* Mikrographie cler Glasbasalte von Hawaii, p. 8 and fig. 30. 



