CHAP. XXVIII.] BURIED UPRIGHT TREES. 137 



instead of soil, they were compelled to engage, in 

 stead, 150 well-practised axe-men from Kentucky. 

 I am informed that the superintendant of the gas 

 works, Dr. Rogers, who is now absent in Cuba, 

 endeavoured to estimate the minimum of time re 

 quired for the growth of the cypress and other trees, 

 superimposed one upon the other, in an upright 

 position, with their roots as they grew, and had 

 come to the opinion, that eighteen centuries must 

 have been required for the accumulation. At the 

 time of my visit the section was too obscure to 

 enable me to verify or criticise these conclusions ; 

 but Mr. Bringier, the State surveyor, told me, that 

 when the great canal, before alluded to, was dug to 

 the depth of nine feet from Lake Pontchartrain, 

 they had cut through a cypress swamp which had 

 evidently filled up gradually, for there were three 

 tiers of the stumps of trees, some of them very old, 

 ranged one above the other ; and some of the trunks 

 must have rotted away to the level of the ground in 

 the swamp before the upper ones grew over them. 

 If it be true, as I suspect from these statements, 

 that the stools of trees which grew in fresh water 

 can be traced down to a level below the Gulf of 

 Mexico, we must conclude that the land has sunk 

 down vertically. Perhaps some part of this sub 

 sidence might arise from the gradual decay or com 

 pression of large masses of wood slowly changing into 

 lignite, for carbonated hydrogen is said to be con 

 stantly given out from the soil here wherever such 

 masses of vegetable matter are decomposing ; and 

 during the excavation of these works much inflam- 



