1895. MICROSCOPICAL JOURNAL 45 



At six or eight feet depth the shells of oysters and other 

 marine animals are often abundant. 



This marine deposit is conclusive proof that all these 

 marshes were once open water; the huge bank of sand 

 that now prevents the waves from flowing over them is 

 a superficial structure lying on top of the marine deposit 

 which projects through it, and shows at low water as a 

 bank of mud some three feet higher than the surface of 

 the water at low tide, and extending downward to a vary- 

 ing depth and outward into the open water ; in some 

 places it can be traced outward for nearly a mile or until 

 the water becomes too deep to follow it. The upper 

 layers of this ancient mud, where it shows on the steep 

 face of the sandy beach, are filled with roots of sedges 

 and contain the same varieties of diatoms as the upper 

 stratum beneath the marsh. In fact this stiff' mud contin- 

 ues in an unbroken sheet from far out in the open sound 

 back to and through the ridge of sand forming the 

 beach and under the entire surface of the marsh. This 

 shows that these marshes were once much more extensive 

 then they are at present, and that the sea has been for a 

 long time at work driving back the sandy beach and 

 washing away the surface of the ancient deposit. This 

 fact introduces an element of uncertainty into our study 

 of recent marine diatoms. 



I have pointed out in previous articles that the pres- 

 ence of even an abundance of fresh water diatoms in a 

 marine deposit, is no proof that fresh water had ever ex- 

 isted near that locality ; the diatom being brought down 

 in abundance during high freshets in creeks and rivers, 

 and distributed over vast areas by the action of tides 

 and storms. It might be supposed that soundings, and 

 gatherings of recent soft muds, would contain only re- 

 cent varieties ; but when we consider that hundreds of 

 acres of ancient deposits have been washed away to a 

 depth of from six to tw^enty feet, and that there is a core- 



