DESCRIPTrON OF ILLUSTKx'iTIOXS. 151 



Fig. L'S. Sli(]e M0'"-''\ Earlier diabase, Siitro TmncI, iiortli IjraiK^li, 50 Icct south of 

 Opliir connection. Nicols crossed. Magnified .'!(• dianicters. 



LABRADOiiiTE: 27-i;i; 27-23; 23-15; 10-27, and most of tlie grains 



constituting the groundmass. 

 Augite: 11-27; 5-20; 21-33; 21-12. 

 UrAlite: 22-8; 7-20. The angite at 21-12 is partly converted to 



uralite. 

 Magnetite or ilmenite: 9-20; 10-12; l(i-7, etc. 



PLATE A^. 



Fig. 20. Slide 46(i"--'. Later diabase ("black dike"). Chollar imne, 1,!I00. foot level. 

 Nicols at 45°. Magnitied 120 diameters. 

 Labradoritk: 12-18, etc. 



Augite: 25-26; 14-7; 14-0; I()-20; 25-18, etc. The augite is all 

 more or less obscured by a smoky-browu decomposition product, 

 probably liuionite. 

 Magnetite: 6-22; 16-18; 23-7, etc. 



Fig. .50. Slide 228"-2''. Earlier hornblende-andesite. Knoll just northeast of Comhi- 

 nation Shaft. Nicols at 45°. Magnified 23 diameters. 

 Hornblende: 27-23. 

 Labradorite: 22-10; 20-17; 17-25. 

 Magnetite and ilmenite: All the black spots. 

 Chlorite: 10-29. 



Ftg. 31. Slide 465'"'^''. Decomposed augite-andesite from Crown Point Ravine. No 

 polarizer was used, the sky light happening to be sutticieiitly polarized 

 to develope the lamelliE of the feldspar. Magnitied 20 diameters. 

 Labradorite: 25-25; 11-11. 



Augite: 19-5; pseudomorphs of chlorite after augite, 7-19; 17-32. 

 In the first of these, epidote is developing as in Fig. 5, which is 

 also from this slide. 

 Epidote: 8-19; 18-10. 

 The mass at 25-7 is chlorite, calcite, epidote, and oxides. The black spots 

 in the groundmass are magnetite. 



Fig. .32. Slide 473"'-^-. Later hornblende-andesite. Quarry 2,000 feet northeast of 

 Sutn) Shaft IIJ. Nicols at 45°. Magnified 35 diameters. 

 Hornblende: 10-18; 27-13; 23-3; 13-21; 14-25, etc. 

 Mica : 19-9 ; 15-30. The last is almost wholly represented by mag- 

 netite, leaving only here and there a particle of the original ma- 

 terial. 

 The one large feldspar and all the microlites appear to be labradorite. 

 The magnetite grains are readily recognizable. 



