. Bulletin of the Geological Society of America. Geology. 178 T. C. EOPKINS—CAMBKO-SILURIAN LIMOXITE ORES coated with manganese oxide. In some instances the lining of the shell is covered with a great many small stalactites of ore, indicating a de- posit on the interior since the shell was formed. Many of the shells are lined with a dense fibrous layer, often an inch or more in thickness. The last two varieties resemble some of the quartz geodes and indicate a similar origin. The thinner shells have nearly all been broken, and we see only the fragments of them in the clay-ore masses. This shel
. Bulletin of the Geological Society of America. Geology. 178 T. C. EOPKINS—CAMBKO-SILURIAN LIMOXITE ORES coated with manganese oxide. In some instances the lining of the shell is covered with a great many small stalactites of ore, indicating a de- posit on the interior since the shell was formed. Many of the shells are lined with a dense fibrous layer, often an inch or more in thickness. The last two varieties resemble some of the quartz geodes and indicate a similar origin. The thinner shells have nearly all been broken, and we see only the fragments of them in the clay-ore masses. This shell form of the ore is common throughout the area and forms an appreciable part of the ore body in many places. The small, irregular, nodular-like pieces of ore, commonly known as shot ore, are presumably closely related in origin to the shells, the difference being that the segregation was around more numerous centers, and hence resulted in smaller Figuee l.—Cross-section f Ore NoduU from Cambrian Slates. PIPE ORE Pipe ore comprises two distinct types, one of which (illustrated by numbers 6, 7, 8,9, and 11 in plate 50, figure 1) consists of heavy compact pipe-like masses from 1 or "I to 8 or 10 inches in diameter, the ore crust being from a fraction of an inch to 3 inches or more in thickness. In nearly all cases this type of pipe ore is impregnated with clay, grains of sand, and other foreign material. The inference is that the iron oxide was deposited around lime stalactites or stalagmites which were subse- quently leached out and carried away. The presence of the sand and foreign material in the oxide suggests that the lime stalactites were im- bedded in the clay and sand when the deposition of the iron took place. The other type of the pipe ores consists of a loose aggregation of slender pipes or rods, all having a general parallelism and each pipe cemented to its neighbors at frequent intervals. The iron oxide is comparatively free from impregnating foreig
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