94 THE MICROSCOPIST. 



organisms, now generally regarded as plants, are exceed- 

 ingly small, and are usually covered by loricse or shields 

 of pure silica, beautifully marked, as if engraved. These 

 loricre or shells having accumulated in great quantities, 

 have given rise to very extensive siliceous strata. Thus 

 the "infusorial earth" of Virginia, on which Richmond 

 and Petersburg are built, is such a deposit eighteen feet 

 in thickness. The polishing material called Tripoli, and 



FIG. 53. 



Fossil Diatomacese, etc., from Mourne Mountain, Ireland: a, a, a, Gaillom lla (Mclo- 

 seira) procera, and G. granulata, </, d, d, G. biseriata (side view); b, 6, Surirella {dicata ; 

 c, S, craticula; k, S, caledonica; e, Goinphonema gracile; /, Cocconeina fusidiuui; g, 

 Tabellaria vulgaris; h, 1'inuularia dactylus; i, P. nobilis; /, Synedra ulna. (From 

 Carpenter.) 



the deposit called in Sweden and Xorway berg-mehl or 

 mountain flour, because used in times of scarcity to mix 

 with flour for bread, are similarly composed. Strata of 

 white rock in the anthracite region of Pennsylvania, and 

 from the sides of the Sierra Nevada and Cascade ranges 

 in California and Oregon, have also been found to consist 



O ' 



of such remains ;Fig. 53). 



