ZIEGLER, THE RAVENSWOOD GRAN0D10RITE 5 



parallel to (001). Several grains of microcliiie are present usually show- 

 ing, between crossed nicols, the characteristic "Scotch-plaid" structure. 

 The plagioclase is of the oligoclase type and is twinned according to the 

 albite law, although some patches are twinned under the pericline law in 

 addition. The bands due to the latter law are commonly the wider. In- 

 clusions of all the other minerals occur. 



The hornblende is a dark green variety, in irregular shreds and grains. 

 Good cleavage is shown parallel to (110) and strong absorption along this. 

 The pleochroism is intense, ranging from dark olive green to light green 

 to yellowish brown. The extinction angle is 21 degrees. 



Light green augite is present. It has weak pleochroism and an extinc- 

 tion angle of 36 degrees. Uralitization has developed, and the greater 

 part of the augite has gone over into green hornblende. This uralite 

 closely resembles the other hornblende in the rock in appearance and in 

 optical properties. Both augite and hornblende seem to be primary min- 

 erals, and the alteration of the augite seems to be the result of the pressure 

 which is also suggested by the wavy extinction and the crushed rims. 



Brown biotite is present in small amounts, some of it as an alteration 

 product of the hornblende. It then spreads along the cracks towards the 

 center of the grains of hornblende. It is also present in small shreds and 

 in lath-shaped crystals and often occurs in beautiful aggregates. 



The garnet in this type of the rock is light red in color, pale pink in 

 thin sections. It is associated with the darker minerals. 



Magnetite and pyrite are usually present in small amounts. Titanite, 

 zircon and apatite are found in all slides. 



Chemical analysis: 



Per cent 



Si0 2 75.61 



A1 2 3 12.53 



Fe 2 3 2.22 



FeO 86 



MgO 20 



CaO 1.35 



K 2 3.18 



Na 2 5.21 



H 2 09 



101.25 



Recasting into the component minerals we obtain 



Per cent 



Quartz 31.62 



Orthoclase 16.68 



Albite 44.02 



