4^4 PHYSICAL GEOGRAPHY. 



as they do so, much of the organic matter of which they arc composed 

 is dissipated, but some of it, along with the hard siliceous valves and 

 connecting membranes which constituted the skeletons of the diatoms, 

 falls to the bottom of the pond, and forms a layer of greater or less 

 thickness, according to the time during which it has been accumulating. 

 If it be exposed now, by draining such a pond, it may appear as a brown 

 or grey powdery mass, but, if it has rested beneath the the water suffi- 

 ciently long, almost all of the organic matter will be removed, and the 

 clean, white siliceous skeletons alone remain. In some localities, — and 

 this I have found to be the case in the state of New Hampshire, perhaps 

 from the peculiar topography of the spots where these masses of the 

 accumulated dead shells of diatoms arc found, — these organisms grow in 

 bogs of no very great superficial extent, but which, from their occurring 

 in hollows between hills, arc often quite deep. Under such circum- 

 stances, as I should judge from the character of a deposit I examined at 

 Bowkcrville in Cheshire count)', the organic matter might for the most 

 part decay out of a layer of considerable thickness, and nothing be left 

 but a mass of finely divided siliceous material of a character well fitted 

 for use as a polishing powder, or for other purposes to which this sub- 

 stance has been applied. 



Such arc the results, then, of this rapid growth of the diatomaceae in 

 ponds, lakes, marshes, and rivers; and, as the first examples of such 

 deposits which I examined were found beneath layers of peat, I gave to 

 them the name "sub-peat" deposits, and under that designation they 

 have been generally known. After a time, however, specimens came 

 into my hands which were procured from the bottoms of existing ponds, 

 and these, besides consisting for the most part of little else than silica, 

 and being of an almost pure white color, had no peat overlying them. 

 Hence, of course, I saw the inapplicability of the term "sub-peat" to 

 such deposits, and for them I have coined a new name, viz., lacustrine 

 sedimentary, which I consider more appropriate, and at the same time 

 indicating their usual origin, and including all deposits of fresh-water 

 diatomaccous remains, with the exception of certain peculiar layers to 

 be hereafter described. Of course the sub-peat then become a variety 

 of these. Deposits of this character arc extremely common in this 

 country, as well as elsewhere, and it will be at once seen that, although 



