H. S. Washington — Ischian Trachytes. 383 



connection, as being very closely analogous to the Ischian forms 

 and an evidence of the existence of internal tension in the 

 crystals of these rocks. 



Rosenbuseh* mentions several cases of similar forms, and 

 they have been described by Iddingsf in a rhyolite of the 

 Eureka District in Nevada. Analogous forms have also been 

 noted by Herz;f in a diabase from Guagua Pichincha, by Vogt§ 

 in a slag, and also by H. Vogelsang. [ 



It is of especial interest, however, to compare with the Ischian 

 forms the branching feldspars observed by Iddingsf in the 

 Obsidian Cliff in the Yellowstone Park, and by Cross** in rhyo- 

 lites of Custer Co., Colorado. The branching feldspars in 

 these cases are orthoclase and are found as constituents of cer- 

 tain spherulites. Some of them are not due to fission, but 

 are branched crystallographically and the prisms are elongated 

 parallel to either the c or d axis in different parts of the group. 

 Others seem from the descriptions and figures to be quite 

 identical in nature with those of Ischia, the prisms and needles 

 being also elongated parallel to a. 



The conclusions to which these two observers come regard- 

 ing the structure and origin of spherulites in general are especi- 

 ally noteworthy. Iddings points out that we must " base the 

 general definition of spherulitic structure on some other charac- 

 ter than outward form," and that this fundamental character- 

 istic is " their mode of crystallization " ; spherulitic growth 

 consisting in " the formation of radiating or diverging groups 

 of crystals," whatever may be their outward shape, or whether 

 the divergence takes place from one or more points or a plane. 



While this definition may perhaps be thought to be in some 

 ways too broad, (since strictly according to it we could call the 

 radiating groups of tourmalines in granites spherulitic), yet 

 that it applies to the larger number of true spherulites, if not 

 all, can hardly be denied. According to this definition 'the 

 Ischian (as well as the Hawaiian) keraunoids are true spheru- 

 lites, though they are not groups of radiating, separate, rami- 

 fied crystals, but one individual. 



They are in fact, as has been said, the " sphoerokrystalle " of 

 Lehmannff and Kosenbusch.Jt This term is not appropriate, 



* Rosenbuseh, Mikr. Phys., i, 36, 628 ; Taf. iii, fig. i : ii, 494, 548. 



f Hague, Geol. of Eureka District, Mon. xx, U. S. G. S., 1892, 378, PL III. 

 fig. 14. 



\ Reiss and Stubel, Reisen in S. Amer. Hochgeb. Rep. Ecuador, I, Berlin, 1892. 

 88. 



§ Vogt. Mineralbild. in Scbmelzmassen, Kristiania. 1892, 1.79, PL II, 15. 



flDie Krystalliten, Bonn, 18*5, Taf. xiv, figs. 4 and 6. 



biddings, Bull. Phil. Soc, Washington, xi, 1891, 445, cf. Obsidian Clifl 

 7, Rep, U. S. G. S, 276 ft 1888. 



** Cross, Bull. Phil. Soc. Washington, xi, 1891, 411. 



\\ Lehmann, op. cit., i, 379. XX Rosenbuseh, op. cit. i, 36. 



