Page:Walcott Cambrian Geology and Paleontology II.djvu/232

 rather abundant and a very few minute grains of quartz, small prisms of apatite, and a few crystals of pyrite can be recognized. Numerous dark brown to black streaks arranged parallel to the cleavage represent carbonaceous matter. There is a system of parallel veinlets less than a millimeter across, which are normal to the slaty cleavage; fractures through the centers of these veins show small grains of calcite and blotches of cupriferous pyrite. The surfaces of a system of later fractures are irregular and are coated with carbonates. Sections of the veinlets mentioned are made up in large part of an isotropic mineral which is nearly colorless in the thin section. In the hand specimen it is pale green. It has an index of refraction of about 1.62 and preliminary chemical tests indicate that it is near the chlorites in composition. A further study of the mineral is being made. In the center of the veinlets are irregular crystals of calcite and a little pyrite.

A chemical analysis of the slate was made by Mr. George Steiger in the laboratory of the United States Geological Survey and is given under No. 1 of the following table. Analyses of several somewhat similar rocks and of a sericite are also given.

1. Middle Cambrian shale from British Columbia.

2. Sericite. Dürrberg. Quoted by Dana, System of Mineralogy, 6th edition, p. 618, analyses 41.

3. Mansfield slate (Lower Huronian). Crystal Falls District, Michigan. U. S. Geological Survey Monograph 36, p. 59.