36 THE AMERICAN MONTHLY [February, 



crawling, performed by pseudopodia. It is, in fact, a somewhat jerky 

 sort of glide. There are, nevertheless, advocates of the existence both 

 of cilia and of pseudopodia as the means of the diatom's propulsion, 

 although no certain glimpse has ever been obtained of either kind of 

 organ. 



Remains. — From the architecture of the typical frustule it may be 

 inferred that whenever, b}- any means, its organic material is destroyed, 

 its two valves will fall apart, and that, if the diatom is in an advanced 

 stage of incomplete subdivision, it will break up into four valves and 

 two hoops. To such constituent parts the microscopist commonly re- 

 duces his diatomaceous material, by treatment with acids, for purposes 

 of permanent mounting and preservation. Nature, also, is constantly 

 removing the soft and perishable endochrome from diatoms which have 

 run their life's course, and is steadily depositing their nearly indestruc- 

 tible remains at the bottom of almost every permanent body of water. 

 As diatoms have swarmed in river, lake, and sea for countless ages, at 

 least since the glacial epoch, their flintv shells have com(i to form beds 

 and strata of very considerable extent in nimierous parts of the world. 

 The fresh-water, salt-water, and brackish-water deposits are very dis- 

 similar in character, as are also the pievailing forms of different periods. 



Deposits. — Amongst the largest and l")est-known deposits are that 

 which underlies tlie city of Richmond. Virginia, and its vicinity, and 

 the one near \'irginia City, Nevada. The '• Richmond earth" forms 

 a stratum of from S to 30 feet in thickness, lying near the surface and 

 extending throughout the eastern part of Virginia and portions of 

 ^Maryland. There is some reason to suppose that this extensive deposit 

 is related to if not actually connected with deposits recently discovered, 

 at depths of several hundred feet, at Atlantic City and other points in 

 Eastern New lersev. The forms prevailing in these deposits are of 

 the kinds peculiar to salt water, and their accumulation, in these im- 

 mense beds of wide extent and great thickness, is evidence of a long- 

 continued submergence of the regions in which they are found beneath 

 an arm of the sea. 



The Nevada deposit is of very pure fresh-water forms, and was laid 

 down in one of two great intra-glacial lakes, which geologists tell us 

 were nearly as large as Lake Superior, one of which filled the Utah and 

 the other the Nevada basin. The Nevada deposit has been worked 

 commercially for a number of years, as the source of the polishing- 

 powder which goes by the trade name of " Electro-Silicon." This and 

 other diatomaceous earths are also employed to some extent, I believe, 

 to form an absorbent base for certain high explosives. Indeed, very 

 many of these earths are valuable articles of commerce. They furnish 

 silica in a finely-divided form, suitable, amongst other things, for the 

 manufacture of the silicate of soda or of potash — otherwise known as 

 " soluble glass" — which is an ingredient in the glazing of pottery, in 

 artificial stones, and in certain cements and paints. 



Growth of our Knowledge of Diatoms. — Under the microscopi- 

 cal powers of early days the valves of all but the very largest species 

 appeared as simple in structure as if made of perfectly plain transparent 

 glass. .Some of the coarsest, particularly thediscoidal forms, displayed 

 upon their surface a dotting or embossing, with occasionally a rayed or 

 more complicated pattern. A few of the angular forms presented a 



