510 PAL&ONTOLOGICAL REPORT OF GEOLOGICAL SURVEY. 



upon it by quiet and shallow waters, will be hardened to black shales, 

 and show us the petrified remains of plants, shells, or fishes. The de- 

 posits of the deep, quiet, marine waters, have formed a bed of lime- 

 stone above it, and if, afterwards, sand has been brought in by the cur- 

 rents of the sea, the whole measures — coal, shales and limestone — 

 become covered with sandstone. 



The only thing not explained above, is the formation of the fire-clay 

 of the bottom, which, by a cross-section, would certainly be found un- 

 der the coal of the Dismal Swamp, as it is found under nearly every 

 bed of the old Coal Measures. 



As we have seen before, the woody matter deposited in a basin can 

 only be preserved and transformed, if the water is of a constant level. 

 Resting on the sand, the water percolates through it, and consequently. 

 is subject, by a constant motion, to a perpetual change of chemical con- 

 stituents, and to a renewal of the particles of air which it contains. 

 This change is opposed to the formation of peat, since water, before 

 being prepared for the preservation and transformation of woody sub- 

 stance, has to become saturated with a peculiar acid — the ulmic acid — _ 

 produced by the decomposition of wood itself. Thence it follows, that 

 a peat or coal basin has to be separated and prepared to keep its water, 

 like a well cemented cistern. This work is done by very small animals 

 — infusoria — and by peculiar species of plants. In the peat forma- 

 tions of the present, day the clay bottom of the bogs is prepared by 

 fresh water molluscs and infusoria, and by the vegetation of the cTiar- 

 acece and confervce, two families of cryptogamous plants, which disap- 

 pear entirely, as soon as the peaty vegetation begins. They fix in their 

 shells, or in their tissue, the carbonate of lime or the silica, 

 abundantly dissolved in some water, and by their decomposition they 

 deposit those substances at the bottom of the water in the form of a 

 very fine mud. In Denmark, there are some perfectly isolated ponds, 

 where this soft mud or clay is formed, by the agency of the above 

 named animals and plants, at the rate of one foot and more in every 

 five years. 



As there is no bed of peat, but is underlaid by soft white clay, so 

 there is no bed of mineral coal without its bottom of fire-clay, except 

 when it has been deprived of it by some accidental circumstance. This 

 fire-clay is free from remains of animals and shells, but it contains very 

 abundantly the stems and leaves of a species of plant, Stigmaria ficai- 



