38 THE CHICAGO ACADEMY OF SCIENCES, 



rite, chalcosite, native copper, etc., and that which produces 

 the green rust (Aerugo nobilis) on old copper domes on build- 

 ings, antique copper coins, and on bronze vessels. 



Azurite. 2 (CuCO 3 )' Cu(OH)o. Monoclinic. 



This form of the copper carbonate, which is less stable than 

 the above described, crystallizes more readily than does mala- 

 chite, and the crystals are usually in short columns or tablets. 

 Its cleavage is complete, parallel to the clinodome. It is 

 brittle, its fracture uneven. Its hardness is 3.5 to 4.0, its gravity 

 3.7 to 3.8. Its luster is glassy, its diaphaneity translucent to 

 opaque. Its color is azure blue, and its streak a lighter blue. 

 Blowpipe reactions similar to those of malachite. It occurs 

 with malachite as an incrustation. Azurite changes readily to 

 malachite, the more stable form, by the loss of CO 2 and by the 

 gain of H 2 O. 



Thus 2 [2(CuC0 3 )' Cu(OH) 2 ] + H 2 = 3 [CuCO- Cu(OH) 2 ] 



+C0 2 . 



SILICATES. 



We now come to a division of the Oxygen Salts composed 

 entirely of emigrants. The number of the species represented 

 in this division is about 15. And these occur in innumerable 

 quantities forming the bulk of the material which buries the 

 Niagara limestone over most of the region to the depth of a 

 hundred feet or more. It is the second division of the Oxygen 

 Salts, and consists of the SILICATES. 



The most abundant representatives of the silicates are 

 found in the FELDSPAR GROUP, comprised of both the mono- 

 clinic and triclinic sections. All of the crystals which I have 

 examined are in various drift boulders. While they do not 

 represent the crystallography of the species when studied 

 macroscopically, they do show other physical characteristics, 

 such as hardness, specific gravity, color, and luster, and when 

 made into thin sections and studied under the microscope, 

 their crystallographic and optical properties can be observed. 

 These different feldspars resemble each other very closely. 

 While they occur in two different crystallographic systems, their 

 external form and their angle of cleavage differ but slightly. 

 The prism angle being nearly 60 and the angle between the 

 base and the pinacoid plane about 90. Their general habit 

 and twinning methods are similar. All have two directions 

 of perfect cleavage, one parallel to the base and the other par- 



