410: PROF. EHRENBERG ON FOSSIL INFUSORIA. 
the stratified structure remains as fully visible in the Polirschiefer 
‘as it had before been, and forms the stripes of the 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-mass, 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 
varians, particularly the larger individuals; 3. Gatllonella ferruginea ; 
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 formed Polirschiefer. Upon heating this yellow 
semi-opal, it became red and actedasiron. 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 Kosemitz in Silesia. The microscopical bodies inclosed in 
this stone, very apparently of a spherical form, and never occurring 
larger, which are also attached externally to the semi-opal or hornstone 
