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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1878 Excerpt:...from 100 to 300 feet in height, with an occasional knob rising higher. Porphyry forms almost the entire substructure of the region, with here and there outcrops of granite. The crystalline rocks were eroded into ridges and valleys before the deposition of the limestone took place. These porphyries are older than the Silurian limestones. Modes of Occurrence of the Lead Ores. A. As float mineral. B. Deposits in the limestone. Float mineral is a term applied by the miners to designate that mode of occurrence, where the galena is found imbedded in the superficial soil immediately overlying the limestone. Occasionally large masses of the galena will be concentrated in a small area only a few feet from the surface, but being so irregularly distributed that the mining is very uncertain. Instances of this mode of occurrence, where the yield was very large, sometimes occur: at one point the miners took out from five to eight thousand pounds to the man, and the masses of ore weighing from a few pounds to several tons. A single piece weighed 7,800 lb. The deposits occurring in the limestone may be subdivided as follows:--I. Disseminated ore. II. Deposits in flat sheets. III. Vertical fissures. IV. Horizontal fissures. I. The disseminated lead ore usually occurs in a stratum of limestone varying from 2 to 6 feet, occasionally reaching a thickness of 15 or 20 feet; the beds of limestone are nearly horizontal, usually having a dip of 8 to 10. Rarely does the inclination become greater. The galena in this formation seldom occurs alone, but is usually associated with iron and copper pyrites; and at some points the latter mineral occurs in considerable quantities, and in a few cases, as at Mine la Motte, nickel and cobalt ores are found. It will be remembered that the Mine...