PROF. EHRKNBERG ON FOSSIL INFUSORIA. 409 



to verj' 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 Crinoideae) 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 microscopic£il analysis of the most varj'ing 

 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 tliis process 



