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44 THE MEDALS OF OREATION. Cuap. IIT. | 
the bones retain their usual quantity of phosphate of lime, 
but the animal matter is converted into carbon. This 
alteration appears to have taken place unconnected with a 
high temperature, and to have been a spontaneous change 
in a moist situation, to which air had no access.* 
Another, and very remarkable condition, is that in which 
the phosphate of lime has been removed by the infiltration 
of water charged with sulphuric or carbonic acid, and the 
gelatin converted into leather by tannin; as is the case 
with bones and teeth of deer, horses, &c. obtained from a 
submerged forest of oak, larch, &c. near Ferry-bridge, in 
Yorkshire ; of which there are many instructive specimens 
in the York Museum.t 
The cancellated structure (that is, the little cells or pores) 
of the long-bones of mammalia, found in caverns in England 
and Germany, and in the breccia of Gibraltar, and the 
conglomerates of Ava and the Sub-Himalaya mountains, 
&c., are often filled with crystallized carbonate of lime. In 
the Wealden deposits the osseous carapaces and plastrons of 
Turtles, and the bones and teeth of Crocodiles, Lizards, &c., 
are almost without exception heavy, and of various shades 
of brown or umber, from the infiltration of solutions of 
carbonates and oxides of iron. 
In some instances, bones of a jet black are imbedded 
in the white calciferous grit ; the phosphoric acid in the 
original organism having combined with iron and produced 
a deep blue or black phosphate of that mineral, and left 
the surrounding stone uncoloured. 
Petrifaction by the infiltration of calcareous solutions is 
equally common ; and the medullary cavities of the bones are 
frequently lined or filled with white calc-spar ; brilliant pyrites 
also enters into the composition of these fossils, frosting over 
with a golden metallic deposit the cavities and fissures. 
* Mr. Smee, London Med. Gazette, November 1840. 
+ Communicated by Professor John Phillips. 
