38 



Zonal structure unquestionably indicates changes in the environment of 

 the crystal during the period of growth. The separation of crystals in a 

 magma must necessarily produce a change in the composition of the part 

 which remains liquid, and this cause alone may explain such cases as those 

 observed by HOPFNER and BECKE. Changes from green to brown, and vice versa, 

 observed in certain hornblendes can probably be explained on the assumption that 

 the growing crystals were alternately subjected to oxidising and reducing 

 agencies. Until we are more fully informed as to the chemical and physical 

 changes which occur during the period of crystal-building in igneous magmas 

 and the effects of such changes on the growing crystals we cannot, however, 

 hope to render a complete account of the various structures observed in the 

 crystalline constituents of igneous rocks. 



The curious " hour-glass " structure first described by WERWEKE (1) may 

 here be referred to, though it is not a zonal structure in the proper sense of 

 the term. In the case of an augite possessing this structure a section parallel 

 to the vertical axis, taken in any azimuth, can be separated into four fields in 

 such a manner that the two opposite are alike in colour and optical properties. 

 The two pairs, however, differ from each other to some extent in respect of 

 these properties. Sections through the crystal at right angles to the axis show 

 merely a zonal structure. This curious phenomenon can be explained on the 

 assumption that a skeleton of augite, having an hour-glass form, with the 

 vertical axis corresponding with the length of the hour glass, was first formed 

 and the crystal completed by the filling up of the space between the two bulbs 

 with augite substance of a somewhat different composition. 



SPHERULITES. Crystalline aggregates of a more or less spherical form, 

 and possessing therefore a certain amount of individuality occasionally occur 

 as constituents of igneous rocks. The surfaces of these aggregates are some- 

 times smooth and sharply defined from the rest of the rock, at other times they 

 are irregular, and the external boundaries are indistinct. 



True spherulites, according to ROSENBUSCH, give a black cross, which 

 remains stationary as the stage is rotated under crossed nicols. They are com- 

 posed of crystalline fibres having each an axis of elasticity coincident with the 

 axis of figure ; the separate fibres being arranged in a radial manner round 

 one or more centres. Such spherulites are especially common in glassy rocks 

 having a high percentage of silica. They usually have a smooth surface 

 and are sharply defined from the glass in which they lie embedded. 



Spherulitic aggregates, as has been already pointed out, are sometimes 

 formed by the intercrystallisation of two or more minerals. These are termed 

 pseudo-spherulites by RoSENBUSCH. Their optical characters are usually more 

 complicated than those of the true spherulites in consequence of the want of 

 coincidence between the axes of elasticity and the axes of figure in the 

 individual elements. The well known orbicular diorite of Corsica (Napoleonite) 

 may be quoted as an illustration on a large scale of this type of spherulite. In 

 this case the minerals constituting the spherulite are anorthite and hornblende. 



Many individual minerals such as epidote, certain zeolites, arragonite, 

 chlorite, &c., tend to form radial or spherulitic aggregates. 



(1) Beitrag zur Kenntuiss der Limburgite. N.J., 1879, pp. 482 and 822. 



