BY HENRY G. STOKES, F.G.S. 
17 
eider whether they correspond in character to the best descrip- 
tions of clays such as kaolin or china-clay, premising that in so 
much as there is a superabundance of clay suitable for both 
pottery and bricks in the immediate neighbourhood of Brisbane, 
the discovery of additional deposits of this class is of little or 
no commercial interest. 
In estimating the suitability of a clay for the manufacture 
of china, the following factors have to be regarded ; — First, as 
to its composition, what quantity of silicate of alumina does it 
contain, and to what extent are impurities associated with this 
silicate ? Secondly, the nature of the treatment required to 
eliminate these impurities? Thirdly, the amount of clay of 
uniform quality available? For these are the questions which 
have to determine its employment as a commercial success or 
otherwise. 
In the case of what is known under the different names of 
kaolin, china clay, porcelain clay, or Cornish clay, we have a 
hydrous silicate of alumina with certain impurities, which 
when the latter are removed has the following average pei'cent- 
age composition, namely: — Silica, 47*2; alumina, 31*1; and 
water, 18*7 ; that is, regarding the silica as wholly combined a 
clay containing 67*33 per cent, of silicate of alumina, and 11 per 
cent, of free silica. The abovementioned impurities, which 
comprise small quantities of lime, iron, alkalies, and (quartz) 
sand, are almost entirely removable by a simple process to which 
the clay is submitted before being placed on the market or con- 
verted into chinaware. In fact it is found that the preparation 
of the clay for the market involves an expenditure of only 17s. 
per ton and that it is sold at prices ranging up to 85s. per ton. 
The Cleveland clay, in the light of the Aldershot analysis, 
quoted in the correspondence to which allusion has already been 
made, contains by analysis 56*60 per cent, of silica and 33*02 
per cent, of alumina, that is, regarding again the alumina as 
, wholly combined, 71*4 per cent of silicate of alumina and 18*22 
I per cent of free silica. Judging from samples which have come 
^ to hand the Cleveland clay also lacks uniformity in its physical 
' characters, such as colour, texture, &c., a condition of things 
' indicative of a variability also in chemical composition. In fact 
