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Lake earth Mr. Stodder writes me as follows : " It was found 

 under a few feet of water, uncovered by any earth — the white 

 siliceous deposit showing through the water." This is rather a 

 remarkable fact, showing that the once superimposed peat has 

 either been removed or changed into diatomaceous earth by the 

 decay and removal of its organic matter. It contains so very lit- 

 tle beside the siliceous shells of the DiatomaccEe, that it requires 

 but once boiling in nitric acid, and elutriation by means of water, 

 previous to mounting in Canada balsam. The species found in it 

 are the followinoj : 



There is a Nitzschia occurring in this deposit, of which, as I 

 have found it in a living state at a different locality, and as I con- 

 sider it as yet undescribed, I shall at some future time speak 

 more fully. 



The Cambridge earth, Mr. Stodder informs me, occurs under 

 the following circumstances : " There is a deep bed of clay, over 

 that a few inches (4 to 6) of white sand, in which I could not 

 find a single Diatom ; over that one or two inches of reddish 

 earth, probably mostly decomposed vegetable matter, and contain- 

 ing the Diatoms and Monad'ina ; over that, vegetable turf and 

 roots. The ground was much broken and disturbed ; the above 

 was as near as I could make it out. The locality is but a foot 

 or two above the level of a neighboring pond, and was no doubt a 

 swamp." The species found in it are : t 



Eunotia serra, Pinnidaria lata, 

 Einiautidium bidens, " major, 



" yracile, '' tabellaria, 



Navicula jirma, ^ " viridis, 



" (/ibberula, Stauroneis ])hoenicenteron, 



Ortliosira oriclialcea, Synedra radians. 



Though the number of species in this deposit is not great, it is 



