1909-10.] 
355 
The Glenboig Fireclay. 
the Carpalla Mine near St Austell, and recognised abundant kaolinite.* 
Scales of kaolinite also occur in the clays of Bovey Tracey and in a sample 
of the Stourbridge clay, though in each case they are mixed with halloysite. 
I have not, however, been able to find any kaolinite in the Glenboig fireclay 
or any Scotch clays. The clay substance in them, so far as I have examined 
them, is either an amorphous silicate or excessively fine particles of quartz. 
The hydrous silicate of alumina in this Glenboig firelay was probably 
formed by the decomposition of felspar by carbonic acid at or near the 
surface of the earth and at low temperatures ; and thus it is not surprising 
that the mineral thus produced has taken the form of an amorphous and 
not of a crystalline silicate. 
Clay substance in general, of course, includes much more than halloysite, 
for it is sometimes silica in the form of quartz flour, or it may be kaolinite 
or the dust of other silicates of alumina ; or fine grained sericite or other 
white mica.*)* Halloysite, however, is probably the essential constituent of 
most common clays. It is the chief, but not, as implied by Senft, the 
universal clay substance. “ Das Grundbildungsmittel fur alle thonartigen 
Substanzen ist die Thonsubstanz,” which, he continues, when pure, consists 
of two molecules of silica, one of alumina, and two of water.| 
That some fireclays are formed of sericite has been proved by Hutchings § 
for some near Newcastle-on-Tyne, and it is well known that much of the 
clayey material which miners have called talc and dolomite is only sericite. 
There is, however, comparatively little white mica in the Glenboig fircelay. 
4. Materials included in the Clay. 
The commonest inclusions in the clay are grains of quartz (fig. 3), which is 
sometimes so abundant that some thin layers become argillaceous sandstone. 
The quartz, however, is in large grains, and it thus acts as a refractory 
constituent, and the clay behaves better as a fireclay than might be inferred 
from a bulk chemical analysis. 
Felspar is fairly common, and includes many grains and cleavage flakes 
of very fresh plagioclase, probably derived from some volcanic rocks that 
existed in the neighbourhood during the formation of the Millstone Grit. 
Hornblende and mica, including biotite, are both present ; but the mica is 
scarce in comparison with the fireclays investigated by Hutchings ; there 
* This identification was subsequently fully confirmed by Mr G. Hickling, “China 
Clay; its Nature and Origin,” Trans. Fed. Inst. Min. Eng., vol. xxxvi., 1909, pp. 1-26, pi. 1. 
t For the presence of quartz flour as a clay constituent, see, e.g., F. Senft, Die Thonsub- 
stanzen, Berlin, 1879 (published 1878), p. 15. 
I F. Senft, ibid., p. 16. 
§ W. M. Hutchings, “ Clays, Shales, and Slates,” Geol. Mag., decade 4, vol. iii., 1896, p. 315. 
