354 Proceedings of the Royal Society of Edinburgh. [Sess. 
cleavages parallel to their hexagonal edges. This hexagonal cleavage was 
described by Reusch * from the kaolinite of the National Belle Mine, 
Silverton, San Juan County, Colorado. The flakes of kaolinite appear 
to be less elastic than those of mica, and accordingly they frequently 
break along the cleavages; flakes are found with sectors broken out by 
fracture along these cleavages. 
Halloysite, on the other hand, has no cleavage ; it is granular and 
amorphous, and often has a faint yellowish tint. It agrees with the 
material described by Mr Hutchings f as the finest substance in the 
Seaton fireclay. He says : “ The thinnest possible films of fireclay under 
Jth-inch objective show a minutely granular substance, with usually a 
faint yellowish tinge and of such an extreme tenuity that in polarised 
light it is quite inactive, or depolarises only just perceptibly in a faint 
speckly manner.” “ This granular matter,” he adds, “ is, I suppose, 
the mixture usually spoken of as kaoline.” This substance may be 
called kaolin if found in sufficient purity to be useable for making 
china clay ; but it is not kaolinite, and when amorphous I should prefer to 
call it halloysite.J; 
Scales of kaolinite occur in various clays. Dr Hatch, § in his Text-book 
of Petrology , correctly stated that kaolinite “is the chief constituent of 
china clay.” I had occasion early last year to examine the china clay from 
* H. Reusch, “ Krystallinische Kaolin von Denver in Colorado,” Neu. Jahrb., 1887, 
vol. ii. p. 71. 
t W. M. Hutchings, “Notes on the Probable Origin of some Slates,” Geol. Mag., New 
Ser., decade 3, vol. vii., 1890, p. 271. 
X The publication of this paper has been delayed to allow some chemical tests as to the 
solubility of the clay substance to be repeated. According to Lacroix* kaolinite is 
“ inattaquable par les acides,” while hydrochloric acid “decompose facilement” halloysite. 
This distinction between the two species seems reasonable, as an amorphous material such 
as halloysite might be expected to be more easily decomposed than a crystalline species such 
as kaolinite. A careful investigation by Mr D. P. Macdonald shows that the clay substance 
of Glenboig fireclay is readily decomposed by boiling in hydrochloric acid. Thus 6*5 
per cent, of the total of 37*65 per cent, of alumina was dissolved out by boiling for two 
hours in hydrochloric acid, 23*2 per cent, by boiling for six hours, and practically the 
whole of it (36*6 per cent, out of 37*65 per cent.) by boiling for thirteen hours. Boiling 
in hydrochloric acid therefore completely decomposes the material ; and if M. Lacroix’s 
distinction be valid, it cannot be kaolinite. 
As the Glenboig fireclay is of Carboniferous age and underlies an intrusive sill, it is 
not surprising that the percentage of water in its clay substance is smaller than in some of 
the more recent French halloysites, and it is therefore somewhat less readily decomposed 
and is nearer in physical properties to kaolinite than varieties of halloysite with a higher 
percentage of water. 
§ F. G. Hatch, Text-book of Petrology, 1892, p. 103. 
Mineralogie de la France et de ses Colonies, vol. i., Paris, 1895, p. 472. 
