312 



FOSSIL DEPOSITS OF DIATOMACE.&. 



whole series being the representative of the Chalk formation of 

 Northern Europe, in which the silex that was probably deposited 

 at first in this form has undergone conversion into flint, by agencies 

 hereafter to be considered (Chaps, x., xix.). Of the Diatomaceous 

 composition of these strata we have a characteristic example in 

 Fig. 144, which represents the Fossil Diatoraacese of Oran in 



Fig. 144. 



Fossil Diatomacece, &c, from Oran: — a, a, a, Coscinodiscus ; 

 b, b, b, Actinocylus ; c, Dictyochya fibula ; d. Lithasteriscus 

 radiatus ; e, Spongolithis acicularis ; /, /, Grammatophora 

 parallela (side view) ; g, g, Granimatophora angulosa (front 

 view). 



Algeria. The so-called ' Infusorial Earth ' of Richmond in Virginia, 

 and that of Bermuda, also Marine deposits, are very celebrated 

 among Microscopists for the number and beauty of the forms they 

 have yielded; the former constitutes a stratum of 18 feet in thick- 

 ness, underlying the whole city, and extending over an area whose 

 limits are not known. Several deposits of more limited extent, 

 and apparently of Fresh-water origin, have been found in our own 

 silands ; as for instance at Dolgelly in North Wales, at South 



