The Pamgenetie Relations of Minerals. 337 



Langenbogen (Prussia) is very probably a product of sublima- 

 tion. The spherical masses of hydrated pyrites, covered 

 with an envelope of gypsum, the crystal points of which are 

 turned inwards, have decomposed with considerable evolution 

 of heat. The sulphate of iron has been washed away, and 

 the masses are now hollow, with the surplus sulphur, result- 

 ing from the decomposition, upon the points of the gypsum 

 crystals. The heating of the pyrites has in this case been so 

 great, that the coal immediately surrounding the gypsum has 

 been converted into a species of anthracite. 



It is not improbable that analogous changes may have 

 taken place in other rocks possessing a porphyritic structure, 

 but the difficulty here is, that we do not know what was the 

 condition of the matrix previous to the formation of the im- 

 bedded crystals. Graphite occurs imbedded in the sandstone 

 of Charlottenbrunn (Siberia), and it is very probably a product 

 of the alteration of organic remains at some period after the 

 formation of the rock. When it is remembered that some of 

 the mixed rocks must be regarded as really metamorphie, 

 the fourth proposition will appear applicable to a much greater 

 number of facts, although it may not always be possible to 

 furnish positive explanations of them. 



5. Certain minerals occur imbedded in the older rocks, 

 only where they are in contact with more recent eruptive 

 rocks. 



Several geologists state that the occurrence of the very 

 frequently associated minerals, kyanite and staurolite, is 

 limited, in the older schistose rocks, to those spots where 

 they are in contact with granite, or some analogous rock. 

 Here, then, the porphyritic separation of crystals is obviously 

 owing to the influence of the more recent upon the older rock. 

 It cannot be doubted that an essential part of the change 

 consisted in a chemical readjustment of the atoms. Anda- 

 lusite, which, according to Bunsen, is specifically identical 

 with chiastolite, likewise occurs in mica and clay slates, under 

 precisely similar circumstances. It has been found very fine 

 in the clay slate of the Whealkind mine, at the surface of 

 contact with granite, the clay slate at the same time being 

 remarkably hard. The same phenomenon presents itself 



