178 FOOTNOTES FROM 



very realms of the air are peopled with diatoms. The 

 atmospherewe breathe contains hundreds of species, which 

 float about on every breeze, and are wafted hither and 

 thither. Many of them remain for years in the highest 

 strata of the atmosphere, until carried down in the full 

 capacity of life to the nourishing waters of the stream 

 and the lake, by descending currents of air. They have 

 been found in immense numbers in the impalpably fine 

 dust, which at certain seasons broods like a thick haze 

 over the island of St. Domingo, and occasionally falls in 

 great quantities on the decks of vessels far out on the 

 Atlantic. The sirocco and trade winds convey immense 

 quantities of them for hundreds of miles. Clouds of 

 diatomaceous dust, giving the atmosphere an orange or 

 ochre hue, have repeatedly been observed coming in vari- 

 ous directions from the coast of Africa, falling on vessels, 

 and diffusing around a darkness so dense as often to 



now living in our waters, and forming deposits which will become rock at 

 some future time ; and as some species are peculiar to lakes and rivers, 

 and others to seas and firths, while some affect deep and others shallow 

 water, these tiny plants are capable of furnishing considerable information 

 to the geologist, with regard to the conditions under which raised sea- 

 beaches and fresh-water limestone rocks were originally deposited, and the 

 circumstances which operated in the production of the different strata in 

 which they occur. I may add, as an illustration of the universal diffusion 

 of these plants, the curious fact, that the late Dr. Gregory found numerous 

 most interesting diatomaceous forms in small fragments of soil not exceed- 

 ing a pinch of snuff, adhering to specimens of exotic plants in herbaria. 

 In every case, without exception, he found these organisms ; and in all, 

 the proportion to the whole non-calcareous earthy residue was wonderfully 

 large. The soils in which the most numerous species were found, were re- 

 spectively obtained from the Sandwich Islands and Lebanon. Many of 

 Ehrenberg's profound observations were made on portions of foreign soil 

 procured in this manner, and his example should stimulate collectors of 

 plants to preserve carefully every vestige of earth adhering to the roots of 

 their exotic specimens, as in this way many new forms may be brought to 

 light, and many rare ones studied in the quiet and leisure of home, with- 

 out the trouble and fatigue of collecting them in their native localities. 



