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 pieces. 



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 foreign material. Somewhat similar forms of 

 lime carbonate have been observed where the waters from a lime spring- 

 trickle over an overhanging, jagged ledge of rock, either on the surface 

 or in a subterranean cavity, and the inference is that the pipe ores have 

 been formed in a somewhat similar manner. They have all been broken 



