GEOLOGICAL AGE AND FORMATION. 47 



always be an island, with a few trees on it. . These layers 

 I found until I came to the beach." " From the channel 

 of Grassy Sound to twenty rods west of the drain on the 

 left, shells were found down below the surface of the mud 

 twelve and fifteen feet." No trees or buried timber of any 

 kind are found in the deep parts of the marsh ; they are 

 not uncommon in the shallow parts near the shores. 



The level of the salt-marshes is nearly that of high- 

 water mark J those portions along the creeks, and other 

 water-courses, are usually a few inches higher than those 

 further back. Where there is no cedar-swamp bottom, the 

 marsh is more solid near the passages through which the 

 tide flows than at a little distance from them ; the differ- 

 ence being due to the greater or less amount of mud in it. 

 In many parts of the marjsh scarcely any mud is to be 

 found, the whole substance consisting of the roots of sedge, 

 reeds or flags, which, when dry, is almost as light as hay, 

 is very combustible, when consumed leaves very little ash, 

 and, when allowed to decay slowly, wastes away to almost 

 nothing. The muddy water which comes in with the tide 

 deposits the principal part of its sediment near the water- 

 courses, where its rapid flow first slackens, and by the time 

 it has reached the back part of the marshes it becomes 

 clear. The banks which are made by the mud hinder the 

 water from draining out completely at low-tide ; and the 

 coarse grasses which thrive in such localities continually 

 increase, and their matted roots, buoyed up by the water, 

 maintain the apparent level of the marsh. This appears 

 to have been the mode of growth throughout ; for in sound- 

 ing the marshes at different depths, variations in solidity 



