1909-10.] The Glenboig Fireclay. 353 
proportion of water left at 100° C. was as low as 15*27 per cent. ; in a 
variety of indianite dried at 100° C. it is 14 per cent., and in a series of 
analyses by Le Chatelier the water left after heating to 250° varies from 
14*3 to 13*0 per cent. One of these samples contained only a total water 
of 16*5 per cent. It seems, therefore, allowable to define halloysite as an 
amorphous bisilicate of alumina with two molecules of combined water, and 
with a few per cent, of water less firmly united. If that definition of 
halloysite be accepted, there is no reason why much of the material called 
“ clay substance,” including the base of the Glenboig fireclay, should not be 
described as halloysite. 
It must be borne in mind that all aluminous clay substance does not 
belong to one mineral species, as the material may belong to one of several 
allied species. The late A. H. Green,* for example, in his Physical Geology , 
remarked regarding pure clay that “it is likely that there are several 
varieties differing from one another in the proportion of silica and the 
amount of water they contain.” Percy, in an earlier and careful discussion 
of the nature of clay substance, left the question unsettled, as he recognised 
that the proportions of water varied in different carefully analysed clays. 
Thus Forchhammer’s analyses show that the china clay of Passau has the 
composition of — 
Silica 46*23 
Alumina ...... 35*27 
Water ...... 18*50 
100*00 
Its formula is 9Si0 2 , 4A1 2 0 3 , 12H 2 0, which differs from kaolinite by an 
excess of both silica and water. The slight excess of silica is probably 
due to some mechanically included fine quartz flour ; and if so, the formula 
of this Passau clay may be represented 2Si0 2 , A1 2 0 8 , 3H 2 0. 
For the reasons already given, a variation of 4 per cent, of water 
may be allowed within the range of a mineral species ; and if the Passau 
clay be amorphous, there is nothing in its composition to prevent its 
inclusion in halloysite. 
The distinctions between kaolinite and halloysite can be readily recognised 
under the microscope. The kaolinite occurs in minute scales which resemble 
white mica, but have a much lower refractive index, which can be recognised 
when tilted flakes are examined under crossed nicols. They often grow, 
moreover, in piles, like heaps of cards of different sizes. The larger flakes 
sometimes show, in addition to the perfect basal cleavage, three intersecting 
* A. H. Green, Physical Geology , 1876, p. 68. 
