SCIENTIFIC SURVEY. 399 



I have heretofore spoken of the value of the microscopic Alj^ac 

 simply with reference to their employment as fertilizing agents. 

 Although I regard this as by fur the most valuable use to which 

 their fossil remains may be put, I should not omit to state, that 

 there are other and by no means unimportant purposes to which 

 they may also be applied. Of these the most important are their 

 employment as polishing powders, in the so-called " metallic lus- 

 tres" and in the manufacture of porcelain. The substance called 

 Tripoli is almost invariably composed of nothing but the silicious 

 skeletons of the diatomaceai. A polishing powder has been pre- 

 pared under this name from the Bluehill deposit already mentioned. 

 I come now to the consideration of another and exceedingly in- 

 teresting portion of my subject, but one on which little at present 

 can bo said, viz : the uses of diatomaceous earths in determining 

 the unsettled questions of geological history. It will readily be 

 conceived, after reflection upon the figures already given, that 

 organisms which exist in such countless numbers, and which multi- 

 ply with such inconceivable rapidity should e.xert a vast inlluencc 

 in promoting geological changes, but have we any key in the char- 

 acters and species of the formations themselves whereby to deter- 

 mine the nature and the duration of the causes which produced 

 them ? May they like other fossils be used in determining the 

 relative age of the several beds in which they occur? From their 

 simple structure, whereby they are better adapted than most or- 

 ganic beings to resist the influence of physical cha'.ges,«and from 

 the fact that but few, if any, fossil species'are known, which have 

 not also been found in a living state, it has been until recently 

 supposed that no such use could be made of them. Deposits from 

 various localities were known to be of undoubtedly diilbrent geo- 

 logical age, but no means seemed apparent by which that age could 

 in all cases be definitely settled. A more extended study <.f the 

 distribution of species now bids fair to furnish the wished lor key, 

 although at' present we can only state what is to be hoped for 

 rather than what may be considered as certainly known. 



Some eighteen months ago, while examining the fossil and recen 

 forms from a rich locality in the vicinity of Providence, K. I., 1 

 was struck by a remarkable fact, of which I have nowhere seen 

 mention made, although a distinguished Philadelp -a m.croscop.st 

 has since informed me that he had also observed a smn ar fact^ 

 The fossil sub-peat deposit referred to contains the usuaUanety 



