THE NATURAL HISTORY SURVKY. 2J 



mascasite was in the form of concretions in the' clay, while 

 the greater part had at one time been contained in the lime- 

 stone fragments in the drift. Within two years the active 

 encroachment of the lake having been retarded by piers built 

 into the water the bed of marcasite was entirely decomposed 

 or buried. 



The peat of the region contains iron sulphide whose presence 

 is evident frequently only after the formation of melanterite. 

 This mineral covers the peat like frost. Likewise even in the 

 protection of the museum marcasite decomposes and passes 

 into other compounds. The Chicago Academy of Sciences, the 

 Field Columbian Museum, and University Museums in the 

 vicinity of Chicago, contain many fine specimens of this min- 

 eral. But they are difficult to preserve, since under the influence 

 of moist air, the sulphide changes to the sulphate with formation 

 of sulphuric acid which corrodes containing boxes. [FeS 2 + 

 8H 2 0+;0 = FeS0 4 7H 8 O+H 2 SO 4 ]. 



Decomposition of these minerals in the field give rise to 

 other minerals whose composition is dependent upon the en- 

 closing rock. In the clay beds and limestones of the region 

 are formed white filaments of melanterite which change into 

 a sulphur yellow buttery mass consisting of melanterite 

 (FeSO 4 + ; H 2 O) copiapite (2 Fe a O 8 '5 SO 3 + i8H 2 O), halo- 

 trichite (FeSO 4 - A1 2 (SO 4 ) 8 + 24H 2 O) gypsum (CaSO 4 + H 2 O), 

 and siderite (FeCO 8 ). The presence of minute gypsum fibers 

 and siderite crystals in the clay is due to such a transformation. 



The best localities for marcasite and pyrite are the lake 

 shore and the river valleys of this region. Large quantities 

 were on the embankment between the Des Plaines river and 

 Chicago Drainage Canal near Willow Springs during the build- 

 ing of the canal. 



CLASS III. SULPHO-SALTS AND CLASS IV. HALOIDS. 



These classes are not represented as far as I know in this 

 region. 



CLASS V. OXIDES. 



Of this class there are at least eight species, of which the 

 most abundant and universal representative in this region, as 

 in all regions, is 

 Quartz. SiO 2 . Hexagonal. 



Quartz is found both as a native and as an emigrant, and 

 both in phanero-crystalline and crypto-crystalline varieties, see 



