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depth, and, becoming only covered with a moderately thick coat, 
would become peat bogs ; and, if left for a long series of time 
totally undisturbed, would, perhaps, form lakes of petroleum, similar 
to that which now exists in the island of Trinidad, a description of 
which has been already given in the Fourteenth Letter. 
When we reflect on the situation of coal, and of the various 
strata which are interposed between its several beds, it must, how- 
ever, be admitted that considerable difficulties still oppose the sa- 
tisfactory explanation of these phenomena. Among these, not the 
least, is the frequent alternation of coal and lime-stone, and which 
undoubtedly best agrees with the idea of immense lakes of sea or 
fresh water undergoing alternate fillings and emptyings : in the 
former state, depositing strata of stone, and in the latter, forming 
beds of decomposing vegetable matter. 
Yours, &c. 
LETTER XXV. 
INQUIRY WHETHER THE VEGETABLE MATTER WAS DEPOSITED AT 
THE DELUGE UNDER CIRCUMSTANCES FAVOURABLE TO ITS CON- 
VERSION TO COAL BITUMEN ALONE NOT FITTED FOR FUEL 
OTHER MATTERS NECESSARY TO BE ADDED PECULIAR ARRANGE- 
MENT OP THE PARTICLES. 
T HE next question which demands consideration is, whether such 
an arrangement of the materials of the flooded surface of the ante- 
diluvian world, as has been assumed in my last Letter, would be 
