Emerson — Corundum and a Graphitic Essonite. 235 



pure essonite. Because of this weathering they have now a 

 dull gray and unattractive appearance, and this is increased by 

 the fact that about half the surface of each crystal face is occu- 

 pied by a dull black mineral in irregular blotches, arranged in 

 a somewhat pegmatitic way. This sometimes increases in 

 amount so as to occupy almost the whole surface of each 

 face of the large crystals, and these faces are then irregular, 

 and in the great mass of aggregated crystals where this is car- 

 ried to the extreme it is continued up to a rather sharp plane 

 beyond which the crystals are quite free from the black min- 

 eral, and this plane passes through the middle of the largest 

 crystals. 



In a section cut through a crystal where the surface was 

 almost all black, the dark color disappeared rather suddenly 

 about half an inch from the surface, but the dark mineral 

 penetrated in narrow wedge-like plates far into the compara- 

 tively pure garnet. 



The mineral is a pure soft graphite, especially in the wedge- 

 like projections last mentioned, or a very line granular mix- 

 ture of grapliite and garnet over the gray areas. 



When a fractured surface on a slide is examined with a lens 

 it has nowhere a homogeneous or crystalline appearance, but 

 looks more like a fine-grained white aplite, and this character 

 is maintained under the microscope which shows a brightly 

 polarizing mosaic in which one can, with difficulty, detect here 

 and there a nonpolarizing grain of garnet. 



Large grains or phenocrysts of twinned calcite are very 

 abundant, and the rest of the field is mostly made up of large 

 feathery aggregates of coarse columnar wollastonite, full of 

 minute blebs of quartz and quite large anhedra of diopside. 



It is remarkable that such large well-formed crystals have 

 so small a fraction of garnet in their composition. It is only 

 at the surface th^t a thin layer of nearly pure garnet can be 

 found. 



Corundum. — The corundum block is from a very pure bed 

 about two and a half inches thick, with the glistening luster of 

 the Ceylon massive emery. It is a dark bine over most of its 

 surface with small irregular patches of pistachio-green. 



The specific gravity of the rock is 3'6tl:. It scratches topaz. 



Under the microscope the rock is made up of stout colorless 

 grains two to four times as long as wide, with straight, dis- 

 tinct, almost cubical fracture lines. 



These grains have a very low polarizing color, gray to white 

 of the first order. They have a much higher index of refrac- 

 tion than the cyanite which is scattered in them, in porphyritic 

 plates. 



