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GEORGE C MATSON 



the fibers, to brown at right angles to them. The elongation of the 

 fibers is parallel to C. Occasionally the yellow serpentine appears 

 granular rather than fibrous. Closely related to the yellow serpentine 

 in optical properties is the nearly clear variety found in the cracks 

 where the original olivine granules were apparently fractured by 

 shearing. The fibers always have the same optical orientation, and 

 stand at right angles to walls of the cracks. 



The green serpentine is frequently traversed by fibers, giving a 

 bright interference color. Radiating from these fibers are others 

 which show little double refraction; inclosed by these are light green 

 patches which have low interference colors. 



The serpentine contains an abundance of magnetite and a car- 

 bonate. The light green patches are almost free from magnetite, 

 except for a small amount of very fine dust. As alteration progresses, 

 the fibers become cleared of magnetite, which is evidently removed 

 by solution, and is taken first from the large cracks where the altera- 

 tion began. During alteration the amount of serpentine gradually 

 diminishes, and the amount of carbonate increases. In some cases 

 a clear isotropic substance is enveloped by serpentine. This sub- 

 stance, which has a low index of refraction, is probably opal, as the 

 formation of opal and magnetite is a frequent accompaniment of the 

 change of iron-bearing olivine to serpentine. The presence of the 

 carbonate is doubtless due, in part, to infiltration of calcite, in part, 

 to the carbonation of magnesia with the freeing of silica. 



Mica.- — The mica occurs in light brown crystals averaging o.o4 mm 

 in diameter. The mica has a weaker absorption than ordinary bio- 

 tite, and probably has the composition of phlogopite. Plates cut at 

 right angles to the cleavage frequently show a rim with a reddish - 

 brown color. The same color often shows along the cleavage planes. 

 This colored rim is probably the result of alteration with oxidation 

 of the ferrous iron. As alteration proceeds, this coloring-matter is 

 removed, and the mica becomes lighter colored than it was originally. 

 Frequently three zones of color can be recognized in the same crystal : 

 an inner yellowish-brown zone, surrounded by a zone of reddish- 

 brown color, and this in turn surrounded by a zone which is almost 

 colorless. These zones are optically continuous; but all differ in 

 pleochroism and double refraction. The pleochroism of the light- 



