NATURAL HISTORY OF THE DIATOMACEvE. 463 



assisting him in the investigation of various subjects, as the matter of the 

 conditions of existence of the simple cell, and, likewise, the former char- 

 acters of certain strata in which they are found in vast numbers. The 

 bearings of this latter subject will occupy our attention in Part Sixth. 



PART SIXTH. 



TlIE DlATOMACE^E AND GEOLOGY. 



The manner in which the diatomacese increase, both by true growth 

 and reproduction, has been described in such detail that it is to be hoped 

 that it is thoroughly understood. At the same time, it can be readily 

 comprehended how, as they secrete, from its solution in the water in 

 which they live, the siliceous material constituting their harder parts, 

 and, as they die, this flinty matter must after a time form a deposit at 

 the bottom of the lake or ocean which they inhabit. We are, then, pre- 

 pared to take into consideration the formation of such deposits, both 

 fresh and salt, and their connection with the science of geology. 



The mode of formation of fresh-water deposits of diatomaceae, as 

 lacustrine sedimentary strata and as fluviatile fossil layers, has been 

 fully described in a paper read by the present writer before the New 

 York Lyceum of Natural History, Nov. 28, 1870, and published in the 

 proceedings of that association, vol. I, p. 109; and the major part of that 

 communication will be given here as embodying about all that is known 

 on that subject, and detailing at the same time the author's ideas with 

 regard to the enormous deposits of fresh-water diatomaceae found spread 

 over many parts of the western states of the North American continent. 



We have seen how the diatomaceae increase by subdivision, so that by 

 this means alone they may multiply extremely rapidly, and a single indi- 

 vidual, by means of its descendants, soon populates a large pond or lake. 

 But while subdivision or true growth has been thus progressing, increase 

 by generation or seeding may have taken place at the same time, and, 

 from each individual in turn, several young may have been brought 

 forth, which would multiply the rate of increase very materially, of 

 course. It is true that the mode of seeding of these organisms is not 

 thoroughly understood ; but we know enough to say that it does occur, 

 and very frequently, and that the number of new individuals thus formed 

 is very great. At the same time, numerous individuals are dying, and, 



