338 [B.N.F.C. 



the still fluent portion of the lava was secreted, coating the walls 

 of such cavities with 'hullite.' This mineral must now be con- 

 sidered for a little, because its presence plays an important part 

 in the structural arrangement of the Chalcedony, and perhaps 

 in the deposition of the silica in that form. Hullite is a black 

 pitch or waxy-like substance, passing in colour to yellow-brown 

 and red-brown. It is soft (H — 2 to 3) and extremely brittle, 

 breaking with a smooth, slightly conchoidal fracture. In thin 

 sections the black specimens show a greenish colour, but a 

 good deal of the hullite in the cavities and veins is more yellow- 

 ish-brown and reddish-brown in colour. This mineral has been 

 described as mammillated and 'minutely stalactitic,' but 

 'minutely reniform' or 'minutely spherical' would be a better 

 description, because the 'stalactites' of hullite, like those of 

 chalcedony, have a fibrous structure, and seem to be 'animated 

 by a kind of crystalline spinal energy.' Very rarely these 

 spherical forms are slightly translucent, and by reflected light 

 appear to glow with a fiery amber colour. The thicker 

 deposits of hullite, coating the sides of the veins in which the 

 chalcedony occurs, are often cracked and fractured, representing 

 on a minute scale the larger cracks or veins of the mother-rock. 

 These small cracks are filled up in the same fashion as the 

 large veins, with chalcedony and other minerals. In chemical 

 composition hullite varies, but it might be described as a basic 

 mineral, composed of aluminium, iron, magnesium, and calcium 

 silicates, containing about 39 per cent, of silica, and about 13 

 per cent, of 'water of hydration.' It has been placed minera- 

 logically as 'near delessite,' but might be said to occupy a posi- 

 tion between celadonite and chlorophgeite, so far as chemical 

 composition is concerned. All of these minerals, however, van- 

 in their composition, and by some mineralogists are not regarded 

 as definite species. The old-fashioned name of 'green earth' 

 seems to be the best title for minerals of this class, although 

 several are not 'green,' and a number not 'earthy.' Fifteen 

 years previous to Professor Cole's investigation of hullite, the 

 latter was carefully examined, for the first time, by E. T. Hard- 

 man and Professor Hull. Hardman came to the conclusion 



