410 PROF. EHnENBERG ON FOSSIL INFUSORIA. 



the stratified structure remains as fuUj-^ visible in the Polirschlefer 

 as it had before been, and forms the stripes of tlie semi-opal. The 

 white and less transparent stripes are mostly well-preserved layers of 

 infusoria. It is not improbable that a dissolving medium may have 

 acted upon the siliceous shells as drops of water or steam act on 

 meal. The parts in contact with it were gradually penetrated, and 

 partly dissolved and changed into opal ; or the penetrating matter, pro- 

 ducing the opal, and which occupies but a small space, has assimilated 

 to itself a greater or less part of the empty siliceous shells. The true 

 wood-opal, in which the woody substance is changed into opal, renders 

 the opinion probable that a peculiar opaline mass has supplanted the 

 decayed and dissolved parts of the woody substance, retaining however its 

 form. We cannot easily imagine the expulsion of the siliceous shield-mass 

 by the opal-ma.ss, and of the latter filling its space : therefore it appears 

 conceivable that the opal may be probably formed from the infusoria 

 shells, simply by water or any other dissolving medium except fluoric 

 acid, just as dough is formed of meal. Unkneaded dough contains stripes 

 of meal, — semi-opal has often stripes of infusoria : both are hydrates. 



In the semi-opal of Bilin and of the valley of Luschitz were visible, 

 inclosed like insects in amber, 1. Gaillonella distans; 2. Gaillonella 

 varimis, particularly the larger individuals ; 3. Gaillonella ferrugiiiea ; 

 4. siliceous needles of sponges. The first is mostly dissolved, at times 

 preserved as principal mass, with the outline rather rounded off", although 

 the connecting medium has quite a glassy appearance. The second 

 is mostly well preserved, but rather rounded off; the third is sometimes 

 well preserved in the buff'-coloured specimens, but on account of its 

 minuteness does not admit of a determining character. The latter 

 however is not unimportant with regard to the question of the action 

 of volcanic agency : it may perhaps have been deposited in the moist 

 parts of the previously fomied Polirschiefer. Upon heating this yellow 

 semi-opal, it became red and acted as iron. The red was the articulated 

 fibres of the Gaillonella : they could not therefore possibly have been 

 heated in the air. The tranquil horizontal stratification of the Polir- 

 schiefer (exhibiting perhaps the yearly or periodical deposition of the 

 layers) speaks also for a neptunian action. Hot vapours of the volcanic 

 neighbourhood might have much contributed to the purifying of the 

 mass, without actual fire. The semi-opal of Bilin removes all doubt as 

 to these organic relations. 



Very similar formations, with inclosed forms of organic origin, 

 were also apparent in the semi-opal from Champigny, that out of the 

 Dolerit from Steinheim near Hanau, and that from the serpentine for- 

 mation of Kosenitz in Silesia. The microscopical bodies inclosed in 

 this stone, very apparently of a spherical form, and neoer occurring 

 larger, which are also attached externally to the semi-opal or hornstone 



