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[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 chlorophseite, so far as chemical 
composition is concerned. All of these minerals, however, vary 
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 
