H. S. Washington — Ischian Trachytes. 381 



small forked orthoclase crystals were also seen in the ground- 

 mass of a black glassy obsidian from the tufa of Monte di 

 Procida, near Cuma ; and Rosenbusch* speaks of similar forms 

 in obsidian from Ponza, though I could find none in my 

 specimens. 



In regard to the origin of these peculiar forms, it will have 

 been evident that they are most certainly not due to twinning, 

 and also that they cannot be referred to skeleton forms con- 

 sequent upon growth along certain crystallographic axes or inter- 

 axes. They must be either an aggregation of individuals about 

 a common central point or axis, or else due to the continuous 

 splitting up and growth of what would be under other circum- 

 stances a compact individual. 



Many cases of such ramified crystals are described and 

 figured by Lehmannf and all the evidence points to the 

 Ischian keraunoids being of this character, and not of the 

 nature of aggregate growths of separate individuals. This is 

 clearly seen on examination of the series ; passing from the 

 most glassy obsidians with small and seldom forked crystals, 

 through the brown glassy trachyte, with individuals which are 

 evidently split and whose diverging needles can clearly be 

 seen to be continuous in substance with the central crystal, to 

 the trachytes with colorless basis, where the forms have grown 

 quite complex but in which the continuity can be made out by 

 careful study. 



According to Lehmann, the cause of this ramification is " the 

 existence of internal tensions which cause the crystals to split 

 here and there at the surface, producing a discontinuity which 



cannot be overcome by further growth The broken 



parts grow independently and so form ramifications from the 

 crystal which are no longer oriented exactly parallel to the 

 main mass." These secondary needles may also split and 

 ramify and thus complex forms result. 



That the existence of such internal tension is a sufficient 

 proximate cause is not to be denied, and the facts as shown by 

 Lehmann and elsewhere apparently prove that it is the true 

 one also. It is, however, more difficult to give the cause of the 

 existence of such tensions ; or to determine whether they are 

 inherent in the crystal during growth under certain conditions, 

 ■or whether they are due to the physical action of the magma 

 on the crystal substance. 



That the conditions were extremely local in their character 

 is to be inferred from the few^ instances of ramification found 

 among the Ischian trachytes, as well as in the Hawaiian basalts 

 to be mentioned presently. It is scarcely possible that the 



* Rosenbusch. Mikr. Phys. II, 5G5, 1887. 



f Lehmann, Molecularphysik, Leipzig, 1888, I, 378 ff. 



