46 GEOLOGY OF THE BRISTOL COALFIELD. 



beds having a large preponderance of sand, of which the following 

 is an analysis — 



Silica ^0.91 



Alumina ------ 10.29 



Peroxide of Iron - - - - 3-87 



Carbonate of Lime - - - - 21.79 



Soluble Alkaline Salts - - - 1.14 



Moisture, &c. ----- 2.00 



100.00 



They have been used for making bricks, but the result of 



burning contains so small a percentage of clay and is so friable, that 



they form a very poor and treacherous building material. The 



fallacy of using such bricks as these has been proved by the effect 



of floods at Baptist Mills. 



Above and below these sandy deposits are very useful clays 



which are good material for pottery and Terra Cotta work. They 



bake into a very delicate and pleasing shade of brown. They do 



not shrink in firing and perfectly preserve the form in which they 



were moulded. Analysis of these marls give — 



I. 2. 



Silica - - ^1.62 - - - 43.38 



Aluminia - 20.40 - - - 26.46 



Oxide of Iron - 7.83 - - - 12.82 



Carbonate of Lime 13.46 - - • 5.13 



Soluble Salts - .57) 



( - - JO. 21 



Moisture, &c. - 6.12 ) 



100.00 



Professor Ramsay thinks that the conglomerate formed the 

 basins of large inland salt water lakes, like the modern Caspian 

 sea and thai in these lakes the superimposed marls were deposited. 



