382 H. S. Washington — Ischian Trachytes. 



formation of skeleton crystals is the primary cause, as Lehmann 

 suggests (p. 390) ; at least in the trachytes in question no traces 

 of skeletal growth are to be seen, and such growth is well 

 known to be rare in the feldspars. 



That fission and ramification took place after the mass had 

 come to rest, is to be inferred from Lehmann's conclusion 

 (p. 380) that a certain degree of viscosity and acceleration of 

 the crystallization is necessary to the process. It is also indi- 

 cated by the extreme delicacy of the forms themselves, which 

 would hardly be able to withstand the action of a moving magma. 



The existence of the easy cleavages parallel to the base and 

 clinopinacoid probably assisted the fission materially ; and, as- 

 the prisms are elongated parallel to the axis a), may explain the 

 symmetrical arrangement about an axis and not on each side of 

 a plane. 



The fringes at the ends of the sanidine phenocrysts seem to be 

 due to the fission of feldspar substance which crystallized out 

 of the surrounding zone and was oriented like its crystal 

 nucleus, rather than to the fission of the phenocrysts themselves, 

 which latter idea the sharp straight edges of the crystals de- 

 cidedly disprove. The horns so often seen on either side may 

 be due to skeletal growth, and their divergence is explained by 

 the expansion due to fission of the enclosed feldspathic matter. 



The forms just described are closely analogous to the " feather 

 forms" assumed by augite in certain Hawaiian basalts described 

 by E. S. Dana,* to whose kindness I am indebted for an exami- 

 nation of the original slides. 



These augitic groups are much larger than the Ischian ortho- 

 clase keraunoids, are generally coarser, and differ as well in 

 being much more curved and complex. Though the nuclear 

 crystal is not, as a rule, as prominent or as constantly present in 

 the Hawaiian forms, there cannot be much doubt but that they 

 are due to the same cause as the Ischian — an internal tension 

 splitting the crystal and subsequent growth enlarging it on these 

 lines. A tendency among the augite keraunoids towards greater 

 development at one end could be observed in many cases, 

 which might be connected with the hemihedrism of pyroxene.f 



It was also interesting to observe among these basalts, especi- 

 ally the finer grained ones, a tendency of the feldspars to fork 

 and form rudimentary keraunoids somewhat resembling, 

 though much coarser than, those seen in No. 553 of Ischia. 

 The presence of feldspar microlites with "curved processes" 

 apparently due to fission, and surrounded by a dark spherulitic 

 zone, in a Hawaiian basalt glass, J is of great interest in this 



*E. S. Dana, this JourDal, xxxvii, 443, 1889. 

 + G-. H. Williams, this Journal, xxviii, 115, 1889. 

 X Dana, op. cit., 451, fig. 5. 



