452 MESSRS. J. PARK AND F. RUTLEY ON THE [Aug. 1 899, 



H^. Omahu. — A pale greyish- to pinkish-white rock, which 

 under a pocket-lens is seen to consist of irregular streaks of pink 

 and white matter, the streaks having a rudely radial disposition 

 about ill- defined centres. The structure is evidently spherulitic, 

 the spherules appearing to range from about | inch in diameter 

 downward. The boundaries of the individual spherulites are 

 not very distinct. A section of this rock under the microscope 

 shows lappet-like shreds of devitrified glass, which in ordinary 

 transmitted light vary from colourless to pale brown or pinkish- 

 brown, and are chiefly devitrified by globulites. These shreds 

 exhibit strain-polarization between crossed nicols, extinguishing 

 when the principal section of one of the nicols is parallel to the general 

 direction of any one of the arms or parts of processes or fingers 

 belonging to the spherulites, the bodies of which do not happen to 

 occur in this particular section. The fingers sometimes bifurcate, 

 and the branches appear, in section, to be rounded at the end?, 

 resembling in outline the lobes of the common sea-mat (Flustra) ; 

 they very commonly show a distinct marginal growth of crystalline- 

 fibrous character, similar to that of ordinary spherulites. The 

 secondary nature of this surface -growth or envelope on similar 

 finger- like portions of spherulites was long ago pointed out by 

 Vogelsang^ in a rhvolite from Tolcsva, near Tokay. The branches 

 or fingers of devitrified glass are, as already mentioned, very irre- 

 gular, and consequently discontinuous in section, part of a branch 

 being often represented by rudely circular, oval, or amoebiform 

 patches. The bodies to which the arms or branches belong are 

 described by Rosenbusch as spherulites with pseudopodium-like 

 processes : a good example is shown in pi. iv, fig. 4, in vol. ii of 

 the 3rd edition of his ' Mikroskopische Physiographic der Massigen 

 Gesteine,' and on p. 596 of the same work these bodies are spoken 

 of as mikrofelsitspharolithe. 



If one of these bodies could be separated from the surrounding 

 colourless spherulitic matter — that is, if one of them, with all its 

 ramifying arms and fingers, could be isolated — it would probably 

 present the appearance of a coralloid growth with irregular branches 

 emanating from a centre, or from what may approximately be 

 regarded as a centre, since the central portion does not appear to 

 be a distinct spherulite, but rather an indistinct aggregate of 

 spherulitic growths with a tendency to a radiate structure about a 

 common centre pervading the mass. This is well shown in the 

 section made from the specimen [109] described on p. 455. 



The spherulites which constitute the remainder of the rock give 

 a distinct dark cross when viewed between crossed nicols, and vary 

 from about y^ to about -^^ ^ inch in diameter. Small nests of 

 quartz are present here and there, but the rock is essentially com- 

 posed of the two varieties of spherulites already described, namely: — 

 the large, brownish, and once-vitreous spherulites with arms, and 

 the small ordinary spherulites which lie between those arms and 



^ ' Die Krystalliten,' 1874, ed. by F. Zirkel, p. 148. 



