and its Spherulitic Crystallization. 10 L 



Chain Spherulites. — An interestiiio; feature is the presence 

 of chain sphernlites along flowage lines, as illustrated in the 

 photograpliic plate fig. A. The white lines are composed of 

 layers of innumerable minute splierulites, while the darker 

 layers between are of clear green glass free from crystalline 

 products. Examination with a lens reveals the fact that 

 almost without exception these lines are composed of a chain, 

 or layer, of single sphernlites. Generally they are so closely 

 crowded that they coalesce to a considerable extent, less often 

 they touch at the circumference, and in some places they 

 appear quite regularly spaced but separated by areas of glass, 

 generally not wider than their own diameter. Viewed with 

 the lens from above, a layer appears like one of minute white 

 pills sifted into the glass, innumerable in number and spread 

 without order, here thickly clustered, tliere more thinly distrib- 

 uted. The average size of these spherulites is 0-5-0'6™'" and 

 they do not vary much from it. Under the microscope in thin 

 section they appear like the spherulites of feldspar seen in acid 

 lavas, so much so that it is almost difficult to believe one is not 

 dealing with a section of rhyolite. Although snow-white in 

 the glass, and also as seen by the microscope with reflected 

 light from the section, they appear of a light leather-brown 

 color by transmitted light. With a low magnitication the cen- 

 tral part of the spherulite looks uniform and homogeneous, the 

 flbrous character appears more evident as the outer boundary 

 is approached, and the outermost zone is composed of distinct, 

 branching, fibroiis rays. With high magnification the central 

 part is also resolved into masses of excessively fine, tenuous, 

 matted fibers. At the center, where these fibers are cut per- 

 pendicularly, they appear as mere points. 



When the structure of these bodies is revealed by the use of 

 crossed nicols, it is seen that they are not formed uniforuily 

 of single fibers radiating from a common center, but rather 

 of groups, or bundles, of fibers somewhat radially divergent, 

 like brooms, or brushes ; closely stellate groupings of which 

 form the spherulites. Thus they repeat on a minute scale the 

 examples shown in B and C of the plate and described in the 

 preceding section. Like other spherulites they show a black 

 cross, which is sometimes in one bundle, or brush, parallel with 

 the planes of the nicols and sometimes, as the spherulite is 

 revolved and a new brush comes into position, is somewhat 

 inclined, proving that in each brush the majority of the 

 minute fibers tend to have orientated positions and thus make 

 a compound skeleton crystal. This is also like the larger ones 

 desci'ibed and figured in the preceding section. 



The light leather-brown color of the sections of these spher- 

 idites is a curious feature. It is strongest in the central areas 



