THE NATURAL HISTORY SURVEY. 21 



tinguishing characteristics become evident. They are coated 

 with blue and green carbonates of copper azurite and malach- 

 ite. The thickness of the coating indicates many years of 

 exposure to carbonated water. In some protected cavities 

 cuprite crystals occur. Some of them contain crystal threads 

 of native silver. Neither cuprite crystals nor crystallized car- 

 bonates nor silver thread are found in pig copper. Further, 

 these specimens bear the marks of glacial transportation. 

 They are often planed and deeply striated, and we may safely 

 conclude that they are emigrant minerals. Copper, sometimes 

 shows octahedral and dodekahedral planes of various sizes on 

 capillary, wiry, arborescent or massive samples. It has no 

 cleavage; its fracture is hackly. It is malleable. In hardness 

 it is similar to silver 2.5 to 3. In gravity, 8.5 to 8.9. In 

 color it is red. It is soluble in nitric acid. 



CLASS II. SULPHIDES. 



The class of sulphides is represented by at least six species 

 in Cook County. Four of these occur in the Niagara limestone. 

 They are as follows: 



Galena. PbS. Regular. 



In the cavities of the local limestone and in the fragments 

 of vein quartz in the drift, are small samples of lead sulphide 

 which illustrate the distribution of the mineral in bath crystal- 

 line and non-crystalline rocks. Its associates are quartz, cal- 

 cite, pyrite, sphalerite, argentite. None of the crystals show 

 planes, but simply cleavage surfaces. In hardness they are 

 2.6. Gravity, 7.4. In color, reddish lead gray, lighter in fine 

 aggregates; streak, grayish black. Luster, metallic and so 

 brilliant that galena is called " the diamond of ores." On 

 charcoal, decrepitates, giving sulphur fumes and a lead globule. 

 Soluble in nitric acid. 



Argentite. Ag 2 S. Regular. 



The silver sulphide is found in connection with galena, and 

 as a sulphurized coating of the silver threads that occur in cop- 

 per specimens. Malleable, flexible, and sectile. Hardness, 

 2.5. Gravity, 7. Color, dark lead grey. Poor lustre. Before 

 the blow-pipe, swells, gives off sulphur, leaves silver bead. 



