ESTUARINE DEPOSITS 665 



The gravel contains glacial pebbles and rolled boulders up to a 

 cubic foot in size. It is not always present. The mud is a marine 

 or tidal deposit like the blue clay which replaces it elsewhere, and 

 from which it differs chiefly in being nearly or quite free from 

 clay. There seems to be a gradual transition from coarse at the 

 bottom to fine at the top. Some of the sands contain fragments of 

 a bryozoan, and many chips of pelecypod shells, including a small 

 Avicula. Foraminifera, spines of echinoderms and sponge spicules 

 also occur. In a section through Caldicot Marsh near Portskewet, 

 fifty feet deep, a bed of marl was encountered 10 feet above the 

 base, which contained a mixture of fresh and brackish water shells, 

 such as Limnaea, Planorbis. Scrohicularia piperata and Cardium 

 edule. Diatoms are common in it, and also remains of Chara. 

 A similar bed occurs near Cardiff, about 19 miles farther south. It 

 occurs at the same depth. Altogether the beds of zone 3 repre- 

 sent a normal estuarine deposit, with a fauna and flora both brack- 

 ish and fresh. 



The lower part of zone 2 has remains of forest trees associated 

 with it. It is covered by blue clay with Foraminifera and other 

 marine organic remains, indicating a subsidence. The upper Peat 

 bed, sometimes the only one, "consists of various plant remains, in- 

 cluding leaves and roots of yellow flags and spores and mycelia of 

 fungi, while its upper surface is strewn with trunks and branches of 

 trees, oak, fir, and birch being the chief. The fir still retains its 

 bark, and the heartwood, when cut, is often found to have pre- 

 served its original color. Some of the wood has been bored by 

 some kind of beetle." {go: 621.) The peat is very pure, though 

 containing occasional sand grains, Foraminifera or sponge spicules. 

 It often contains an abundance of spherules of iron pyrites, and 

 sometimes the vegetable cells are filled with it, these occupying the 

 place of the departed protoplasm within the resistant cell walls. 

 The origin of this pyrite is similar to that of coast marshes or 

 swamps in general. When the salt water comes in contact with the 

 decaying vegetable matter a series of reactions occurs, which will 

 end in the formation of iron pyrites. The upper clay contains 

 Foraminifera, sponge spicules and other marine organisms, besides 

 disseminated vegetal material. 



2. The Marginal Lagoon or Barachois. These names are ap- 

 plied to the water bodies cut off from the main portion of the sea 

 by the formation of barrier beaches. So long as the lagoon is not 

 filled in by silt and organic deposits, it belongs to the neritic zone 

 of the littoral district. The process by which it is converted into 



