THE MICROSCOPE IN GEOLOGY. IOJ 



tenth to consist of the shells of infusoria, giving a 

 mass of animal remains amounting to 22,885 cubic 

 feet in bulk, and weighing forty tons, as the quantity 

 annually deposited there. How vast, how utterly in- 

 comprehensible, then, must be the number of once 

 living beings, whose remains have in the lapse of time 

 accumulated." Richmond, in Virginia, is built upon 

 a stratum, the so-called " infusorial earth," which is 

 eighteen feet thick, and extends over a wide area ; 

 this is found to consist principally of the siliceous 

 shells of Diatomaceae. 



The mountain meal (bergh-mehl) of Norway, Lap- 

 land, Saxony, sometimes forming a stratum nearly 

 thirty feet thick, is similarly composed. Most of these 

 deposits consist of marine forms of Diatomaceae, but 

 in our own islands, as at Dolgelly in North Wales, 

 Mourne Mountain in Ireland, Mull in Scotland, depo- 

 sits of fresh-water origin have apparently been formed. 

 In the Foraminifera the skeleton usually consists of a 

 many-chambered calcareous shell investing a jelly-like 

 body ; many of these are perforated with numerous 

 little apertures. In the Polycystina, an allied group of 

 the same rhizopod type of animal life, the investing 

 shell is perforated with very large apertures, and it is 

 siliceous ; " the apertures are often so large and nu- 

 merous that the solid portion of the shell forms little 

 more than a network, thus indicating a transition to 

 the succeeding group, the Porifera, or sponges. The 

 Polycystina possess wonderful beauty, and are capital 

 objects for the binocular microscope ; its stereoscopic 

 perfection, as Dr. Carpenter remarks, causing them 

 to be presented to the mind's eye in complete relief, 

 so as to bring out, with the most marvellous and 

 beautiful effect, all their delicate sculpture. 



The Polycystina are probably as widely diffused 

 as the Foraminifera ; they have been brought up by 



