212 REV. J. MILNE CURRAN. 



am acquainted is to be found on the Blayney-Carcoar Road about 

 a quarter of a mile from Mr. Pratt's Residence. In hand speci- 

 mens, the rock shows a rich blue-black colour on freshly fractured 

 surfaces. It breaks easily into small irregular masses. On 

 weathered surfaces, a brown crust is noticeable which can easily 

 be detached. Groups of sphaerulites are at once recognised stud- 

 ding the rock thickly in some parts. Many of these are a quarter 

 of an inch in diameter, and, on polishing the specimens, their radial 

 character becomes apparent. The rock is easily fused to a black 

 enamel. Its powder is not magnetic. I treated a sample in con- 

 centrated hydrochloric acid for five days, keeping it just below 

 boiling-point for twelve hours. Small quantities of flocculent 

 silica were separated and on completion of the treatment I found 

 that a residue of 82*5 per cent was left. 



According to many authors, the rock should be called a " hyalo- 

 melane." But penologists seem inclined to abandon the distinc- 

 tion between tachylytes and hyalomelanes, based on their solu- 

 bility in acids.* 



Slice 47. — Tachylyte, Carcoar, New South Wales. This is an 

 exceedingly difficult rock to slice and in the thinnest sections the 

 sphserulitic bodies remain opaque. When examined under the 

 microscope each sphrerulite is seen surrounded by a crystallization 

 halo. This structure is admirably illustrated in every slice I have 

 made of the rock. When the edges of these sphserulites are 

 examined under a high power they remind one of the rolling mar- 

 gins of cumulus clouds. The minute bodies seem first to have 

 gathered together into little masses and then to have been drawn 

 by the neighbouring larger masses into one great aggregation, 

 leaving the surrounding magma quite free from every individual- 

 ized substance. By transmitted light, the rock is a rich brownish- 

 red and the spaces cleared by the crystallization halos a bright 

 yellow. All the cleared spaces have nuclei except when the 

 section has just cut the outer zone of a halo. As already stated, I 



* J. W. Judd and G. A. Cole — On the Basalt-glass of the West Isles of 

 Scotland. Quart. Journ. Geol. Soc, 1883, Vol. xxxix., p. 444. 



