Overburden.--The upper part of this consists of soil, or “meat earth”; this is usually removed and carefully preserved. Underneath is a hard, often stony or sandy layer, which, in districts where tin is worked, often contains enough tin to pay for washing. With this stony or sandy layer, is usually a considerable thickness of discoloured clay suitable for brick-making. Branches.--These are stony veins which run through the clay stopes in various directions. Sometimes they are quite worthless; but in a few instances they are veritable tin lodes, and contain enough tin to pay for stamping and dressing. Thus at Carclaze, near St. Austell, each 1000 tons of clay yields something like one ton of oxide of tin, and formerly the proportion was much greater. The proportions of these waste materials, as compared with the fine clay procured, are thus stated:-- For every 1 ton of fine clay there is removed--from 3 to 7 tons of sand, average about 3½ tons; from 2 to 5 cwt. of coarse mica, average 3 cwt.; from 1 to 3 cwt. of fine mica, average 2 cwt.; from 0 to 1 cwt. of stones, average ¼ cwt. A cubic fathom of clay rock, of average quality, will yield about 2½ tons of fine clay; and about half a fathom of overburden must be removed to get it. _Suggested Improvements in Preparing._--Collins thinks that there is still much room for improvement in the preparation of china clay, but that such must be a growth of time and circumstances. At the present time, about one ton of water has to be driven off from each ton of clay in the “dry,” and this uses at least 2 cwt. of coals on an average, and costs from 8_d._ to 10_d._ in labour. In a few modern drys, a small economy in fuel has been effected, by lengthening the kiln; but in none has it been brought so low as 1½ cwt. to the ton of clay. Stocker, in 1862, suggested the use of filter beds, and also devised a centrifugal dryer; but neither of these contrivances has come into use, and the first would seem quite inapplicable on account of the extreme fineness of the particles of clay, and the impermeability of even a thin layer of that substance. Some economy might perhaps result from the use of hydraulic filters of calico, such as are used in the potteries for drying the slip; but it is very doubtful if any saving would be effected, as the labour would be about the same, and, against the 2_s._ a ton for fuel, would have to be placed the wear and tear of the calico.