34 THE CHICAGO ACADEMY OF SCIENCES. 



scum on the surface of the water and as a rusty yellowish de- 

 posit on the bottom of small ponds. Occasional fragments in 

 the drift. 



CLASS VL OXYGEN SALTS. 



The class of Oxygen Salts is represented by several species 

 in each of the groups of carbonates, silicates, phosphates and 

 sulphates. Five carbonates occur in greater or less abundance. 



CARBONATES. 

 Calcite. Ca CO 3 . Hexagonal. 



The most abundant and characteristic carbonate of the 

 region is calcium carbonate. It is found in a great variety of 

 forms and associations. The pure calcium carbonate with 

 varying quantities of magnesium and iron vicariously replac- 

 ing the calcium and with various mechanical impurities such 

 as clay and sand mixed through the mass, forms the floor rock 

 of this region. It is of great economic importance as the 

 source of building stone, constructional- material, and road 

 metal. In the limestone are found veins of the pure calcite 

 which illustrate the crystal form and the cleavage. The com- 

 monest forms are those known as u nail head spar," " dog-tooth 

 spar " and the rhombohedral cleavage pieces. PL VIII, fig. I 

 shows six scalenohedral crystals of " dog-tooth spar," found at 

 Stony Island. The prevailing form is that of the most com- 

 monly occurring scalenohedron p(2i3i)R 3 (Fig. 13.) These 

 planes are usually rounded by repetition of the scalenohedrons 

 and the rhombohedrons. Often the positive rhombohedron R 

 bevels the scalenohedron. The negative rhombohedron (oil2), 

 ^ R,(Fig. 14) occurs in multitudes of sharply formed crystals in 

 cavities in limestone. Fig. 15 represents the crystal number two 



Fig. 13. Fig. 14. Fig- J 5- 



from the right of the photograph PI. VIII, fig. I. It is about S/ 8 

 of an inch high. The scalenohedron planes are deeply pitted 

 so that goniometric measurements lack accuracy. The planes 



