380 H. S. Washington — Ischian Trachytes. 



For such divergent crystal forms, which, as will presently be 

 seen, are due to the ramification and growth of a single indi- 

 vidual, and which correspond to the sphozrokry stable of Lehmann 

 and Kosenbusch, I would propose the 'name keraunoid (Gr. 

 fcepavvos, a thunderbolt). This word, which may seem some- 

 what fanciful, is chosen on account of the narrow waist and 

 divergent, equally developed ends, with symmetry about an 

 axis which give them a striking resemblance to ancient Greek 

 representations of the thunderbolt of Olympian Zeus. It must 

 be understood that the mode of formation is connoted with 

 the term. 



Besides these well-defined forms are seen in smaller number 

 oblong spots of about the same size, of a dark gray color and 

 dusty appearance, showing a very fine indistinct structure of 

 parallel or slightly divergent lines. Under high powers these 

 are resolved into bundles of colorless orthoclase needles, less 

 clearly cut than the others, and with such a large proportion of 

 interstitial trichitic basis that their true character is masked. 

 It is probable that these are less well developed forms, as they 

 show a great resemblance to the fringes at the ends of the sani- 

 dine phenocrysts. A few small clusters of needles radiating 

 from a point are also seen — evidently sections cut through the 

 ends of kerannoids at large angles with the axis. 



The extinction of the needles is parallel or only slightly 

 inclined, so that as the stage is revolved a rather broad dark 

 band swings across the keraunoids, with occasionally, in the 

 thicker ones, a short bar at right angles. In consequence of 

 the approximate parallelism of the keraunoids due to flow- 

 structnre, the field under crossed nicols has an appearance simu- 

 lating that of many spherulitic ground masses, being sprinkled 

 with short, thick, black bands lying parallel to one another, 

 with here and there a thick elongated cross. 



An examination of all the other Ischian. slides in my posses- 

 sion (about seventy-five) revealed the presence of the kerau- 

 noids in only two of the blocks of black trachyte from the 

 tufas of the Scanella Cliff, on the southwest coast, where fine 

 sections of interstratified tufa and lava beds are exposed by 

 wave action. One of these shows scarcely any phenocrysts, and 

 the colorless glass basis is rather dusty. The keraunoids here 

 are made up of much finer needles, and with more interstitial 

 trichitic basis than in the Mt. Rotaro specimens, but the 

 structure is quite the same. The other is much more like a 

 normal trachyte in its groundmass, which is almost holocrystal- 

 line. True keraunoids are wanting, but many of the ground- 

 mass orthoclase crystals show divergent forms ; and this may 

 be held to be a type representative of the holocrystalline 

 development of the glassy trachytes described above. Some 



