io6 



THE MICROSCOPE. 



these remains consist for the most part of the siliceous 

 shells of the Diatomaceae, at one time supposed to be 

 of animal origin, but now of undoubted vegetable 

 nature ; in other cases, enormous deposits are found 

 to consist principally of the shells of the Foraminifera, 

 minute animals of low organisation. Chalk hills are 



Forms of Diatomaceae. 



formed almost entirely of the remains of these little 

 creatures, whose shells are often of most exquisite 

 forms. Ehrenberg has computed that a cubic inch 

 of chalk may contain the remains of a million 

 of these creatures. "The Paris basin, 180 miles 

 long and averaging 90 in breadth, abounds in in- 

 fusoria and other siliceous remains. Ehrenberg, on 

 examining the immense deposit of mud at the har- 

 bour of Wismar, Mecklenburg-Schwerin, found one- 



