378 R. 8. Washington — Ischian Trachytes. 



not inclosing crystals. A fine globulitic dust occurs in it to 

 some extent and there is a marked tendency toward splierulitic 

 or axiolitic forms, with a faintly developed radiated structure. 

 This gives rise to rather mammillary or botryoidal borders 

 around the feldspars, of which it has been attempted to give 

 an idea in fig. 1, <?, d, e. 



The last Mt. Rotaro specimen to be described (No. 523) is 

 properly a trachyte, not an obsidian, and it may be mentioned 

 here that the majority of the blocks of Mt. Rotaro are of 

 this rock or the black obsidian. It is dark-brownish black in 

 color, of a dull luster and quite compact. There are very 

 many white glassy sanidine phenocrysts, varying in diameter 

 from 5 to 10 mm . 



Under the microscope the phenocrysts are seen to be the 

 same as in the preceding types, though augite, biotite and 

 magnetite are rather more rare. The sanidine phenocrysts 

 show a much greater development of the fringed borders than 

 in the last case, and the fringe not infrequently reaches a depth 

 of 05 mm , being usually the deeper the smaller the crystal. 

 In some a horn is seen on each side, being a narrow 

 continuation of the feldspar substance, with the fringe of 

 bright needles and interstitial trichites and dust between 

 them. These horns are sometimes slightly bent, diverging 

 outward, and give the crystal the appearance of a shark's egg- 

 sac (" sea-purse"). The sides also show occasionally a narrow 

 border of fine needles and trichites, the needles lying parallel 

 or nearly so to the crystal edge. 



The grounclmass consists of a colorless glass basis quite 

 thickly sprinkled with small black grains and globulites. Its 

 most striking feature is the great abundance of small sheaves 

 of orthoclase needles, a development of the forms just described. 

 These sheaves show a well marked flow-structure, in a given 

 area, the long axes lying approximately parallel. 



In length they vary from 0*2 to 0*5 mm , comparatively few 

 being either longer or shorter, and their width at the widest 

 part is about half their length. While varying considerably 

 in details, yet the general structure is much the same. Some 

 typical forms are shown in fig. 2, a-h, though the great deli- 

 cacy and complexity in most cases is only roughly given. 



These sheaves are composed in general of a single, straight, 

 untwinned crystal ; which at its center is narrow, but which at 

 the ends is split up, the split portions diverging but preserv- 

 ing their continuity with the main body. The fission and diver- 

 gence have gone on, as a rule, approximately equally at both 

 ends, and in all directions around the axis, producing quite 

 symmetrical forms. Besides the diverging needles which are 

 obviously connected with the waist or main crystal, are others 



