456 MESSES. J. PAEK AI?D P. EXTTLEY ON THE [Aug. 1 899, 



which shows no distinct dark cross between crossed nicols, but 

 merely a number of dark brushes, which, on rotation, inosculate, 

 forming irregular kaleidoscopic patterns of black shadowy brushes on 

 a brown ground. This nuclear portion is succeeded by similar but 

 more elongated spherulitic growths which merge into arms, at 

 times more or less straight, but generally curved. The latter usually 

 terminate abruptly in rounded ends, but it is evident that they 

 often ramify for some distance through the rock, transverse or 

 oblique sections of such arms frequently occurring in the slides of 

 thpse Omahu rocks. What may be termed the groundmass consists 

 of the small ordinary spherulites of later development. PI. XXXIII, 

 fig. 2, is an ideal representation of one of the large brown micro- 

 felsite-spherulites, as it would probably appear if isolated from the 

 surrounding material and magnified from 10 to 20 diameters. 

 PI. XXXIII, fig. 1, gives a fair idea of the general appearance of the 

 microfelsitic matter which constitutes these brown spherulites. 

 The drawing represents a portion of one of the arms magnified 

 1100 diameters. 



It will be seen that the microfelsitic matter is largely composed 

 of hazily-defined fibres which often anastomose. They do not show- 

 sharp boundaries as a rule, and small scales and globulites occur in 

 the fibrous mass. This microfelsite appears to be a brownish glass 

 having an incipient and peculiar devitrification which seems to be 

 incomplete.^ Vogelsang describes similar spherulites in the Tolcsva 

 rock as '■apart from the trichites, felsitically devitrified ; in the clear 

 interstices there is still much homogeneous glass, although this 

 appears to be obscured by granular, rusty yellow ferrite.'^ It would 

 appear that the brown microfelsite-sjjherulites represent segrega- 

 tions of a material difi'ering, to some extent, chemically from the 

 rest of the magma. That they were the first bodies, after the 

 trichites, to separate from the magma seems uncertain, since they 

 occasionally envelop crystals of felspar, but as the latter are often 

 corroded it may be doubted whether they are authigenic. Be this 

 as it may, there can be little question that these microfelsite-sphe- 

 rulites represent the earliest portions of the magma that solidified 

 as glass. At what period devitrification took place in them there 

 seems also little doubt, since it denotes a rudimentary kind of 

 crystalline structure of radiating character, more or less ill-defined 

 in the centre of the spherulite, as indicated by the frequent dis- 

 tortion of the dark cross in the spherical forms without arms, and 

 by the very irregular brushes in the armed forms. The arms of 

 these spherulites, when seen in longitudinal section, show a rude 

 parallelism of the microfelsitic fibres. The latter correspond in 

 general direction with the trend of the arms, and in such sections 

 show a rather weak double refraction. It may, however, be remarked 



^ In very tbin sections of the arms of these spherulites barely any change of 

 tint is perceptible when a Klein's plate is used, especially in transverse sections 

 of the arms. This seems to confirm Vogelsang's statement that much un- 

 altered glass is still present. 



2 ' Die Krystalliten,' 1874, p. 149. 



