THE NATURAL HISTORY SURVEY. 3! 



are its brittleness; its hardness of 3.5; its specific gravity of 6, 

 its cochineal red color; its brown red streak; and its adaman- 

 tine luster. 



Corundum. A1 8 O 3 Hexagonal. 



Andrews, Marcy, and others have reported the occurrence of frag- 

 ments of corundum in the form of impure rubies in the lake shore sands. 

 I have been unable to find specimens. The reported finds may have 

 been fragments of clear garnets. 



Hematite. Fe 8 O 3 Hexagonal. 



This form of the iron oxide occurs as one of the phases in 

 the transformation undergone by iron compounds, notably 

 magnetite. It is found here in plates and incrustations. 

 When in massive pieces, such as occur in the drift, it is often 

 micaceous or compact. Red ochreous varieties occur in balls 

 in the clay. When in hard brown argillaceous masses it is 

 known as clay iron stone. It exhibits a conchoidal fracture, 

 a hardness of 5.5 to 6.5, and a specific gravity of 5.3. Its col- 

 ors are red, grey, or black. Its streak is cherry red to brown- 

 ish red. Its luster is metallic or earthly. It is soluble in 

 hydrochloric acid and before the blowpipe yields a magnetic 

 globule. It is the great paint of nature, and gives color to 

 the many rocks of the region. There are no deposits here 

 that are concentrated enough to be of practical value. 



Ilmenlte. FeTiO., Hexagonal. 



Very commonly with other iron and titanium ores there 

 are found in the drift crystals of ilmenite. Their hosts, 

 are usually diorite and gabbro, in which the ilmenite assumes 

 the place of magnetite. It may be separated from the other 

 lighter minerals in the sand by panning. Its hardness is 6; its 

 specific gravity 5; its luster and color similar to magnetite; its 

 streak brown and submetallic thus differing from that of mag- 

 netite; it is opaque in the thinnest microscopic sections. It 

 is slightly magnetic. The best means of distinguishing it 

 from the iron oxides are the chemical tests. With borax in 

 the reducing flame its bead is brownish red. Tin added to the 

 bead changes it to a violet red. 



While the four last mentioned species are unimportant 

 representatives of the minerals of the Chicago area, the follow- 

 ing mineral may rank as one of the leading species: 



