DETAILED DESCRIPTION OF SLIDES. 93 



GKANULAR DIORITE. 

 Slide 213. Bullion Ravine, at Water Company's flume. 



Typical diorite with green fibrous hornblende. This is the tjpical diorite of MoUUt 



Davidson. Macroscopically, it is gray in color and granitic in structure. 

 The sUde shows that it is composed of a mass of crystalHne grains, fiUing 

 the whole space and without the most distant approach to a porphyritic 

 structure. It contains triclinic feldspar, fibrous hornblende, quartz, magnet- 

 ite, a few fragments of mica, and a number of accessory minerals. The 

 hornblende is present only in fibrous crystalline masses and patches, which 

 seem to have crystallized after the feldspar. Many of the masses of horn- 

 blende are cut at right angles to the main axis, and show excellent cleavages 

 at the characteristic angles. It is strongly dichroitic, giving tints varying 

 from buff to sea-green It polarizes with great brilliancy, showing the 

 whole range of prismatic colors. The angles of extinction observed reached 

 20°. In parts of the slide the hornblende is decomposed, the products 

 being chlorite, epidote, quartz, and calcite. The disposition of the original 

 mineral is so irregular that the process of decomposition cannot be studied 

 to advantage. 



The feldspars seem to be without exception polysynthetic plagioclases. 

 The twin striations are irregular in width, but very continuous and sharply 

 defined. The angles of extinction of the twins, which extinguish light at 

 equal incHnations to the plane of the Nicols, are large. Very many such 

 were observed to exceed 20°, and one or two reach 29°. The feldspar is 

 therefore in the main labradorite, and I saw no indications of the presence 

 of any other feldspar species. There are no untwinned feldspars or feld- 

 spathic microlites. Besides the twins following the law of albite, there are 

 many instances of additional periclinic twinning. In several crystals there 

 is well-developed zonal structure. The feldspars are for the most part very 

 free from inclusions of any kind, and are clear and transparent. 



Many grains of quartz are present, but I observed no crystal faces. 

 The quartzes are full of fluid inclusions, some of them dihexahedral. One 

 of these is so large that the movement of the bubble can be clearly seen 

 with a magnifying power of 60 diameters. The bubbles of these inclusions 



