PROF. EHRENBERG ON FOSSIL INFUSORIA. 409 
to very important observations. It consists of a small collection of 
minerals from Bilin, made by Dr. Stolz of Bilin, of a larger one by 
Dr. Reuss, and also of a great number of specimens collected by M. 
von Humboldt. A careful geognostical drawing by Dr. Reuss explains 
the position of the rock-masses of that district. 
The infusoria rock of Bilin forms the upper layer (fourteen feet deep) 
of the Tripelberg, which (differing from the Kritschelberg, with which 
it was formerly confounded) is elevated about 300 feet above the level 
of the brook Biela. It lies on a bed of clay, which is superincumbent 
to the chalkmarl. Beneath these gneiss is found, as the base of all the 
minerals of that district. The upper masses of stone lie west of the 
Tripelberg on a projected mass of. basalt, which forms the Spitalberg, 
and on the other side of which (west) Grobkalk, with many discernible 
petrifactions of small chalk sea animals (many Crinoidez) lie on the 
gneiss. The firmer masses (Saugschiefer and Semi-opal) lie in the 
Polirschiefer towards the exterior upper part, the earthy below, dis- 
posed often without order in layers, the inferior ones being almost ho- 
rizontal. 
The particular attention paid to the Saugschiefer and semi-opal, whose 
numerous transitions were exposed to view, has now given the scarcely 
unexpected result that these also are in the closest connection with the 
infusoria.—The Saugschiefer is, upon microscopical observation, plainly 
only a Polirschiefer, whose infusoria shells are cemented by and filled 
with a formless siliceous matter, just as there are fossil shells both filled 
and empty: this produces its greater specific weight, and all its other 
characters. In the gradual transitions to the semi-opal we see how 
the cement has increased at the expense of the infusoria shells, while 
the small shells have decreased in quantity and in sharpness of out- 
line. 
The formation of the semi-opal in the Polirschiefer appears to be this, 
that it lies imbedded in it in nodules, in the most minute transitions from 
the Saugschiefer. A close microscopical analysis of the most varying 
semi-opals from Bilin, and the neighbouring valley of Luschitz, has 
shown that all these stone nodules, which sometimes equal flint in hard- 
ness and give sparks, consist partly of infusorial forms held together 
by a small quantity of transparent siliceous cement, and partly contain 
inclosed within them single infusoria, but of a larger size, just as amber 
contains insects. It is often very plainly to be seen, that the disposition 
of the Polirschiefer has not otherwise been altered, either by its change 
into Saugschiefer (cemented and permeated by amorphous siliceous 
matter), or by its change into semi-opal, than that by some means a part 
of the infusoria shells, particularly the more delicate ones, have been 
eaten away or dissolved, with which another part, especially of the 
larger forms, has been covered in an unaltered sfate. In this process 
