DETAILED DESCRIPTION OF SLIDES. 105 



The mica is in part decomposed into quartz and epidote. One scale, 

 18-28, happens to be so exactly in the plane of the slide as to show no ti-ace 

 of dichroism. This scale gives an almost absolutely constant interference 

 cross, and is optically negative. It is therefore biotite. The hornblendes, 

 which are much less numerous than the micas, are wholly decomposed to 

 chlorite and epidote. 



The large feldspars are all striated Several of them are cut in the 

 zone at right angles to ooPdb and show lamellae extinguishing at equal 

 angles on each side of the twinning plane. These angles coiTespond to 

 labradorite. One feldspar, which shows both albitic and periclinic twin- 

 nino-, gives angles of extinction which differ by 75° in two successive 

 lamellge, but the angle on one side of the twinning plane is 8° larger 

 than that on the other. The crystal is cut in the zone go P<» and go Poo , 

 and of this zone so little is known that the crystal cannot be pronounced 

 anoilhite. One of the feldspars contains a fluid inclusion with an active 

 bubble. The grains of feldspar in the groundmass are not well preserved, 

 but almost all those in which the angle of extinction is determinable trans- 

 mit least light when the twinning plane is parallel to the plane of the Nicols. 

 It seems probable, therefore, that they are oligoclase. 



The groundmass is composed chiefly of partially decomposed feldspar 

 microlites and much secondary quartz, with some calcite. There is a con- 

 siderable amount of titanic iron in characteristic forms, accompanied by 

 much leucoxene. This decomposition product has the famiHar want of struc- 

 ture close to the undecomposed ilmenite, but the edges of the patches show 

 a granular crystaUine arrangement, as if the smaller particles gradually 

 united into comparatively large ones. The same appearance is often visible 

 in epidote. There are further many colorless apatites, and an unusual 

 quantity of zircons, which draw attention by their relief, and the brilliant 

 colors which they exhibit between crossed Nicols. 



Slide 172. Sutro Tunnel, 20,424 to 20,434 feet from entrance. 



This is a mica-diorite entirely similar to slide 101, except that it con- 

 tains large quartzes, in which are sinuous bays of groundmass. These 

 quartzes contain fluid inclusions with active bubbles. 



