Prof. Ehrenberg on Fossil Infusoria. 87 



matter), or their transition into semi-opal, than that some portion 

 of the infusory shells, especially the more delicate, was corroded 

 and dissolved, and that it included, in an unaltered condition, 

 another portion of the infusory animals, and more particularly 

 the larger forms. The slaty structure of the polishing slate has 

 not been obliterated by this process, for it is apparent in the 

 striped character of the semi-opal. The white and less trans- 

 parent stripes are chiefly layers of well preserved infusoria. A 

 dissolving substance has acted on the siliceous shells like drops 

 of water, or like steam on a mass of flour. The portions in con- 

 tact have been tranquilly penetrated, and they have partly been 

 gradually dissolved and converted into opal ; or the penetrating 

 material that formed the opal, occupying but little space of it- 

 self, has assimilated a larger or smaller part of the empty sili- 

 ceous shells. It is probable that in the true wood-opal, in which 

 the substance of the wood is converted into opal, a particular 

 opaline mass replaced the decayed and dissolved vegetable mat- 

 ter, while the form was still preserved. We cannot so easily con- 

 ceive a penetration by the opaline matter of the spaces oc- 

 cupied by the siliceous shields of the animals; but the idea 

 seems admissible, that by means of water, or some other 

 solvent, but not fluoric acid, opal was formed like a paste from 

 flour. Unkneaded dough has flour stripes, and in the same 

 manner semi-opal has stripes of infusory animals. Both are 

 hydrates. 



\ In the semi-opal of Bilin, and the valley of Luschitz, we can 

 recognise the following included organic bodies : 1. Gaillonella 

 distans ; 2. Gaillonella varians, especially the larger individuals; 

 3. Gaillonellajerruginea ; 4. Siliceous spicula of sponges. The 

 first is for the most part entirely dissolved, but is sometimes con- 

 tained as the chief part of the mass, somewhat rounded in its out- 

 line, although the uniting basis seems quite glassy. The second, 

 also somewhat blunted in its outline, is generally distinctly pre- 

 served. The third is sometimes well preserved in the yellow 

 specimens, but, on account of its minuteness, is not one of the 

 best characterized. This last, which probably occurred in moist 

 portions of the already formed polishing slate, is not unimportant 

 in reference to the question of the influence of volcanic agency. 

 When the yellow opal is heated, it becomes red, and comports 



