340 The Pamgenetic Relations of Minerals. 



Gora (Poland), are remarkable, inasmuch as they are hollow, 

 and lined with crystals, like amethyst balls. There can be 

 no doubt that the formation of these crystals was attended by 

 a contraction of volume, and commenced from the periphery 

 of the nodules. 



Nodular masses of. limestone are very common in transition 

 rocks, especially slate ; at Obernitz (Thuringia), they are so 

 numerous as apparently to form beds. Marl strata some- 

 times contain nodules of very hard limestone or dolomite, 

 traversed by veins of calcite, perhaps accompanied by celes- 

 tine. Argillaceous spharosiderite would appear to have been 

 formed by a segregation of carbonate of iron, after the for- 

 mation of sedimentary rocks. Freiesleben states likewise, 

 that the crystalline spathic iron of the Mansfeld district has 

 in some places an oolitic structure. It must, however, be ex- 

 pressly stated, that both limestone and spharosiderite, when 

 exposed to the atmospheric influence, assume a nodular struc- 

 ture, and this remark applies also to some sandstones, grau- 

 wackes, &c, 



Heavy spar occurs in nodules in clay ; yellow iron ore in 

 the brown coal formation of Bohemia. At Miask (Siberia), 

 the variety of chrysophan, called xanthophillite, occurs in 

 nodules, generally surrounded by magnetic iron ; the crystals 

 point towards the interior, which is either hollow, or filled 

 with a light green mineral resembling serpentine. Nodules of 

 green epidote occur in a gray compact limestone, in the Banat. 

 Quartz occurs in nodular masses, more rarely crystalline than 

 uncrystalline. Crystalline nodules of quartz, which cannot 

 be regarded as detrital, occur in the mica-slate of the Erzge- 

 birge. The Jura limestone at Eichstadt (Bavaria) contains 

 balls of hornstone, sometimes of considerable size, presenting 

 in their interior concentric-coloured streaks, and even con- 

 centric layers and fissures, the latter containing crystallised 

 quartz. It is further interesting to observe, that the greater 

 number of fossils in the chalk consist of flint, and that the 

 larger nodules of this substance are particularly rich in ani- 

 mal remains. It is probable that the formerly existing 

 organic matter had some share in determining the segrega- 

 tion of the silica, as it appears collected round the fossils. 





