384 H. /S. Washington — Ischian Trachytes. 



as Cohen* and Crossf have pointed out ; since a true, or even 

 approximately, spherical shape is an extreme form, and one 

 very rarely attained by the process. For this reason I sug- 

 gested the use of the term keraunoid, which better fits the 

 majority of the forms, and since the need of some term for this 

 type of radiate growth seems to exist. Cross (loc. cit.) denies 

 to such forms the right to be called spherulites, but if Iddings' 

 definition be accepted they properly come under this head ; as 

 it is the radiate manner of growth, and not the number of indi- 

 viduals or the outer form which is the determining classifica- 

 tory characteristic. 



One can infer from the observations of these two writers, as 

 well as from those of others and from general considerations, 

 that true spherulites in this sense may be produced by three 

 distinct processes. These are : by the divergence of many 

 separate, simple, prismatic crystals from a common point or 

 points — the most common type, especially in the smaller spheru- 

 lites ; by the crystallographic branching of radiating prisms, as 

 seen in Iddings's " porous " and Cross's " hollow " spherulites ; 

 or by ramification, which may take place in only one crystal, as 

 in the examples of Cross and Iddings. We might distinguish 

 the two forms of this third type by the use of Tschermak'sJ 

 terms monosomatic and polysomatic applied to meteoric chon- 

 drules. The same terms may be used for the "branched" 

 spherulites ; while those formed by the first named process are, 

 in the nature of the case, always polysomatic. Any two, or 

 all three, of th^ese processes may combine, simultaneously or 

 successively, to form complex spherulites. 



In regard to the mode of growth of spherulites, Qross comes 

 to the conclusion that, antecedent to the crystallization of the 

 feldspar, there was a development of colloidal substance within 

 the area of the spherulite ; and that the fission of the crystals 

 is due to the tension assumed to exist in this solidifying col- 

 loidal mass. That such a tension external to the crystal would 

 induce strains within it, or at least aid the process of fission, 

 seems highly probable. But apart from this an internal tension 

 must exist in the crystal, as shown by Lehmann, and as several 

 facts noted above (such as the curvature of the needles) go to 

 prove. 



In connection with this idea of the formation of a colloidal 

 substance the presence of a zone of dark brown, feebly polariz- 

 ing substance around individual crystals, and in patches con- 

 taining many forked crystals, in the Ischian trachytes, (and in 



* Cohen, Gotting. gel. Anzeigen, 1886, 915. 



f Cross, op. cit., 432. 



X Cf. Cohen, Meteoritenkunde, Stuttgart, 1894, i, 260. 



