24 JOURNAL OF THE [January, 



•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, and I understand that at least one 

 business house in this city makes a specialty of the trade in them. 

 They furnish silica in a fi-nely-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. 



But it is not because of their practical usefulness that diato- 

 maceous earths are of interest to us. Our attention is now fixed 

 upon diatoms merely as objects of scientific investigation and 

 study. Such they have been, in varying measure, in both their 

 recent and their fossil forms, from almost the dawn of microscopi- 

 cal science, a little over two centuries ago, — observers having been 

 attracted to them mainly by the beauty and variety of their 

 shapes, of which you will presently be enabled to judge for your- 

 selves, by means of the photographs which will be thrown upon 

 the screen. 



Under the microscopical 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 the discoidal forms, displayed upon their surface a dot- 

 ting or embossing, with occasionally a rayed or more complicated 

 pattern. A few of the angular forms presented a structure of 

 coarse hexagonal netting, while some of the elongated and boat- 

 shaped kinds were seen to be possessed of stout ribs running at 

 right angles to the median line, or keel. At a later period, not 

 only was the mere magnifying power of the microscope greatly 

 increased, but, what proved to be of much more importance, 



