THE NATURAL HISTORY SURVEY. 3Q 



allel to the clinopinacoid (brachypinacoid) and nearly at 90 

 degrees to each other. Their hardness is 6.0 to 6.5, their spe- 

 cific gravity 2.5 to 2.9. Their color is 'usually light. When 

 pure they are colorless, and their common coloring matter is 

 some salt of iron or copper or titanium, which makes them 

 various shades of yellow, green or red. They melt before the 

 blowpipe to a clear glass and color the flame in accordance 

 with their composition. All are aluminum silicates of sodium, 

 potassium or calcium, and sometimes barium. I have detected 

 no barium in Cook County samples. 



The principal part of igneous rocks consists of feldspar; 

 and clays, shales and slates consist of pulverized and decom- 

 posed feldspar. Optical and blowpipe tests show the follow- 

 ing species in the rocks of this region: 



Orthoclase KAlSi 3 O 8 Monoclinic 



Soda Orthoclase KNaALSi-Oo Monoclinic 



o > o 



Microcline KNaAlSi 3 O 8 Triclinic 

 Plagioclase 



Oligoclase ) n NaAlSi 8 O 8 ) . . 



Labradorite C m CaAl 2 Si 2 O 8 f 



Orthoclase. 



Some of the best specimens of the orthoclase occur in a 

 garnetiferous granite found in boulders at the brick yard near 

 South Evanston. The crystals are white and but slightly 

 decomposed. They show even cleavage planes which have a 

 pearly to vitreous luster. Under the microscope Carlsbad 

 twins are evident, and the crystals show but slight kaoliniza- 

 tion. In specimens from the lake shore decomposition has pro- 

 ceeded so far that the optical properties are obscured. The 

 mineral has low relief and interference colors lower than 

 quartz, because of its slight refraction. On fresh surfaces the 

 hardness is 6. Its specific gravity is 2.5. The specimens are 

 well examined by blowpipe tests. By holding some of the 

 finely pulverized mineral on a platinum wire in the Bunsen 

 flame and repeatedly moistening with hydrochloric acid, if 

 calcium is present it will appear as a green flame through green 

 glass. No calcium is present in the orthoclase under examina- 

 tion. Some of the powdered mineral is mixed with an equal 

 volume of gypsum, when through a blue glass potassium pres- 

 ent will give a violet colored flame. In this orthoclase the 

 'potassium flame is very strong. The sodium flame is orange 



