FOSSIL ANIMALCULES. 557 



1136. In the Polierschiefer are found hard nodules of an 

 Opaline character, which are pretty obviously composed of the 

 siliceous shields of Gaillonellce, united together by an inorganic 

 cement, formed by the partial solution of some portions of the 

 mass, just in the same manner as compact limestones may be 

 formed by the solidification of coral-sand ( 1105). By the simi- 

 larity in the character of these nodules to the Semi-opal of older 

 formations, Ehrenberg was led to examine various specimens of 

 this mineral ; and he has succeeded in obtaining distinct organic 

 remains of animalcules from their interior : so that it is probable 

 that its nodules have been entirely composed, like those of the 

 Polierschiefer, of the siliceous remains of Animalcules, which have 

 subsequently undergone a partial metamorphosis. In the white 

 and opaque bands of Chalk Flints, he has also found spherical 

 bodies, which he considers to be of organic origin ; these are 

 especially abundant in the white siliceous crust which forms the 

 exterior of the flints, and in the mealy siliceous powder some- 

 times found within their cavities. As the same flints often 

 exhibit remains of the structure of Sponges, it would not seem 

 improbable, that the Animalcules may have been drawn into the 

 canals of the Sponge whilst alive, and may have thus been im- 

 bedded within it. It is not a little interesting to remark that, 

 whilst the microscopic contents of more recent strata are all fresh- 

 water Infusoria, those of the chalk are species which must, or at 

 least can. live in the waters of the ocean. It is also a curious 

 and important fact, that, of about eighty species of fossil Infu- 

 soria, discovered in various strata, almost half are species which 

 still exist in the waters ; and thus these forms of life, so long 

 overlooked as invisible specks of brute matter, have a constancy 

 and durability through the revolutions of the earth's surface, 

 which is denied to animals of more conspicuous size and higher 

 organisation, the continuance of whose existence depends upon 

 a greater variety of favourable conditions. 



