POST-GLACIAL HISTORY OF BOSTON. 461 



lower bed is also indicated by the fact that this sediment is half 

 composed of shells, while the upper, colder fauna bed contains much 

 more mud in proportion to the number of shells. On the whole it may 

 be considered as probable that the accumulation of silt here had been 

 at least no faster than that by the Mississippi at present. This 

 great age for the fish-weir finds corroboration in the preservation of 

 the wood itself. It is considerably carbonized; its surface is almost 

 black, and either wet or dry, is very brittle. Fragments of this 

 fish-weir are preserved in a wet condition in Peabody Museum of 

 Harvard University and in a dry state in the offices of the Boston 

 Transit Company. Its preservation seems poorer than that of wood, 

 1500 years old, from a sacrificial well in Yucatan, and every bit as 

 poor as that of wood from the ancient pile dwellings of Lake Neuchatel, 

 Switzerland. The Swiss dwellings had practically disappeared before 

 Roman times, 2000 years ago. 



Yet other factors must enter into our consideration of the age of 

 this fish-weir. At present a wooden pile exposed above the mud or 

 sand is cut off within a few years, by the borings of mollusks. We 

 must suppose, however, that the accumulation of the lower three or 

 four feet of sediments, one-half of the mass of which consists of shells, 

 must have been a slow process requiring more than a few years. 

 Possibly such boring forms were absent from the Bay at that time, for 

 none have been noted in the collections. It is usually held, too, 

 that marine waters cause a more rapid decomposition of wood than 

 fresh waters, not only " by reason of the abundance and variety of the 

 attacking animal types, but also, it is said (Challenger reports: Deep- 

 sea deposits, p. 256) on account of the greater amounts of sulphates 

 and carbonates in sea-water, which by decomposition in the presence 

 of organic acids facilitate the oxidation (destruction) of the plant 

 tissue." This is corroborated by the fact that the deep sea dredgings 

 yield vegetable remains in quantity only comparatively near lands; 

 also that limestones (usually deposited far from land) contain slight or 

 no records of land vegetation (8). These are, however, records of the 

 open ocean, not of a partially enclosed bay, where the water would 

 be brackish, the preservative powers of which would be still further 

 increased by the flow and ebb of the tides. The preservation of wood 

 in fossil deposits of a brackish water origin is exceedingly common; 

 but all such wood is supposed to have been buried within a com- 

 paratively few years. In the case of the fish-weir the fossil shells 

 indicate that parts of it extended above the encroaching sediment 

 during the time necessary to deposit three or four feet of shells and 



