462 PHYSICAL GEOGRAPHY. 



eaten as a delicacy." The natives of our western coast, as well as the 

 inhabitants of some parts of South America, use an "infusorial earth" as 

 a pigment to decorate their bodies. In guano, doubtless the diatomaceos 

 play a very important part, when that substance is employed as a fertil- 

 izer and spread upon our fields, for they then present the silica in an 

 extremely minute state of division to the moisture of the soil and the air, 

 which gains admittance thereto, either along with the water or on account 

 of the porosity of the earth. It has been found that under these circum- 

 stances the silica is dissolved and absorbed by the plant that requires it, 

 in whose tissues it is deposited to form a strong support to its frame- 

 work. The cereals especially require a certain amount of silica, as is 

 well known, for the strengthening of the stem which serves to elevate 

 the seed where it gets the benefit of the sun and the air. So we find 

 that all the grasses, as wheat, oats, sugar-cane, maize, grow best on a 

 soil from which they can abstract sufficient silica for the purpose indi- 

 cated. Instances have come to my knowledge where recent wet deposits 

 of diatomacea:, especially those containing organic matter, and men- 

 tioned above under the designation of lacustrine sedimentary, have 

 proved of real value as fertilizers, when mixed with stable manure and 

 used for cereals, but, of course, they would be objectionable if applied 

 to root, fruit, or leaf crops. 



Many deposits of diatomaccx arc called tripoli and polishing pow- 

 ders ; and these names indicate that they are possessed of properties 

 which peculiarly fit them for polishing hard surfaces, such as metal. 

 The extremely minute state of division of the silica in the diatom-valves, 

 and the readiness with which those valves are fractured and broken down 

 into still smaller angular portions, are remarkable, and could hardly be 

 imitated by any artificially prepared powder. It has been suggested that 

 the vast diatomaceous deposits found in some parts of the world, as the 

 strata occurring in Virginia and California, might be turned to account, 

 as presenting silica in a fine state of division, so that it can readily be 

 acted upon by the alkali, and the so-called "soluble glass" made there- 

 from. One manufacturer has experimented somewhat in this direction, 

 but with what result is at present unknown. 



It is, however, to the scientific student that the diatomacens are of the 

 greatest interest and really of use, for they have proved valuable in 



