The Paragenetic Relations of Minerals. 331 



times it is difficult, without collateral evidence, to tell which 

 part of a pseudomorphous mineral was formed first, as in 

 the case of rhombohedrons of compact brown iron ore, result- 

 ing from the alteration of spathic iron, and covered with a 

 thick crust of malachite. The latter, however, being a deriva- 

 tive of copper pyrites must be more recent than the brown iron 

 ore, because in the same lode copper pyrites is found im- 

 planted upon spathic iron. 



2. The occurrence in sedimentary strata of crystals, pre- 

 senting such sharpness in their edges that they have ob- 

 viously not been deposited as detritus, but formed upon the 

 spot. — Near Meissen, sharply-defined crystals of iron pyrites 

 occur in clay, together with water-worn fragments of quartz, 

 and generally contain a nucleus of carbon. It is probable 

 that they have been produced from ferrugineous solutions by 

 the reducing agency of carbonaceous matter. 



Where large groups or masses of iron pyrites occur im- 

 bedded in stratified rocks, a curvature of the lines of strati- 

 fication may frequently be observed immediately around them, 

 to all appearance resulting from the formation of the pyrites 

 on the spot. In the brown coal-formation, iron pyrites occurs 

 chiefly where the coal-seams are in contact with large masses 

 of clay. As might be expected, a carbonaceous nucleus can- 

 not always be found in iron pyrites, but a nucleus of hepatic 

 pyrites is frequent, and this contains sulphuret of carbon. 

 Again, other than carbonaceous nuclei occur in iron pyrites. 

 In the alluvium at Meronitz (Bohemia), the iron pyrites con- 

 tains nuclei of pyrope, and in perfectly developed dodecahe- 

 drons of pyrites, nuclei of transparent quartz have been 

 found. 



This porphyritic formation of iron pyrites sufficiently proves 

 the segregation of particular substances in rocks. It is not 

 confined merely to deposits of clay, but may be observed even 

 in the oldest schistose rocks. In some varieties of clay slate, 

 the imbedded crystals of iron pyrites are covered, especially 

 on one side, by a layer of fibrous quartz, which must have 

 been deposited after the formation of the pyrites ; and this 

 fact furnishes additional evidence that these two minerals 

 have an attraction for each other. The Devonian slates of 



