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Proceedings of the Royal Society of Edinburgh. [Sess. 
may be seen standing on edge, as if they had fallen through water into soft 
mud and remained imbedded in their original vertical position (Plate fig. 4). 
Microscopic examination of the clay shows that its grains are often very 
fresh and angular. Many of the flakes of felspar have their cleavage angles 
quite sharp, and some of the felspar is surprisingly fresh. 
3. The “ Clay-Substance ” — Hallo ysite not Kaolinite. 
The chief constituent of the fireclay is a fine-grained clay substance, 
containing grains and flakes of various other minerals. 
The chemical composition of the clay substance in the Glenboig fireclay 
has been determined through a careful investigation by Professor Fawsitt 
of Sydney University. He isolated from the clay by repeated washing 
a hydrous silicate of alumina, which is white in colour, and according to 
his analysis is practically identical in composition with kaolinite, though 
it may contain more combined water. But when the water has been driven 
from this mineral at 105° C. the remaining water is in the same proportion 
as in kaolinite, and there would be nothing in composition to show that this 
material when dried at 105° C. is not kaolinite. But microscopic examina- 
tion shows that it is amorphous and not crystalline. 
This material, though it could only be obtained from fireclay at an 
expenditure of labour that would make it a very costly commodity, could 
be used for making porcelain ; so it may be called china clay or kaolin. 
I prefer not to call it kaolinite, since it occurs as amorphous granules, and 
not in crystalline scales. It occurs in minute rounded granules, of which 
the average diameter is about '001 mm. It is one of those forms of clay- 
substance which is not kaolinite. If this material is to be referred to a 
particular mineral species, it may be included in halloysite, the amorphous 
hydrous bisilicate of alumina. The only difference in composition between 
kaolinite and halloysite is in the proportion of water: whereas kaolinite 
has 14 - 0 per cent., halloysite may have up to 20 per cent. But the water in 
halloysite in excess of the two molecules in kaolinite appears to be present 
in a much looser combination, and most of it may be driven off at practically 
the boiling-point. Thus Professor Liversidge * has described a halloysite 
from Berrima in New South Wales with silica 45 per cent., alumina 38*5 
per cent., combined water 12‘8 per cent., and water given off at 105° C. 
3 per cent. This halloysite has practically no more water than samples of 
halloysite, of which analyses are quoted by Dana f : in one of them the 
* A. Liversidge, Minerals of New South Wales, 1888, p. 195. 
+ Dana, System of Mineralogy, 6th edit., 1892, pp. 688-689. 
