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action alteration amount angle appears bands batholith bedding Belmont biotite blocks body Butte calcite cent changes character cleavage closely composition conclusion considerable considered consist cover Creek crystals deposits depth detail determined developed dikes diopside diorite direction distance district Drumlummon east erosion evidence exposed extensive fact fault feet feldspar fissures flat formation fracture Geol geologic giving granite greater Gulch hill hornblende hornstone igneous indicating infiltration intrusion invasion irregular later less limestone limits locality magma margin Marysville mass metamorphic zone metamorphism metasomatic method microdiorite mile minerals nature normal noted observed occur original outcrop parallel Pear places planes porphyry portions possible present probably quartz region relations result rock seen separate sericite shale sheets shown side siliceous slopes strata structure surface surrounding Survey taken thickness tion vein vertical walls zone
Сторінка 105 - In general it may be said that all the veins of the district carry rich ores in bonanzas and ore shoots within the first 200 feet from the surface, but that in depth the ores rapidly decrease in value until the vein is no longer workable. It may also be said that the ore shoots were well defined, and the intervening vein matter barren and unworkable. The pitch of the ore shoots conforms to the usual habit, dipping to the right when looking down the dip of the vein. The ores consist of sulphides and...
Сторінка 104 - Weed," who says: In addition to the pneumatolitic deposite on igneous contacts and those in altered strata near the contacts there are many productive mines working true veins cutting the igneous rock and the contact rocks above them. Such vein fissures are caused both by the contraction due to the crystallization and cooling of the igneous rock and by the shrinkage of the metamorphic zone above the igneous rock. Examples of this type have already been mentioned. As shown by Pirsson...
Сторінка 169 - ... probable that at no great depth the granite underlies the greater part of the district. If the granite merely broke through and involved the original rocks of the area it now occupies, their entire absence from it as inclusions is remarkable; if they had been carried away by fresh accessions from below they should be found as inclusions in certain localities preserved from erosion. On the supposition that they have sunk as fast as freed, the absence of inclusions may be readily explained. If,...
Сторінка 31 - The Helena limestone formation is composed of more or less impure bluish-gray and gray limestone, in thick layers, which weathers to a buff and in many places to a light gray color. Irregular bands of broken oolitic and concretionary limestone occur at various horizons. Bands of dark and gray silicious shale and greenish and purplish argillaceous shale are interbedded in the limestones. These bands are from half an inch to several feet in thickness. There are also beds of thinner bedded limestones,...
Сторінка 105 - This vein, which is the largest and the most productive in the district, consists in its lower levels of a mass of angular rubbish, derived from the walls of the fissure, and in places cemented by quartz, in other places still retaining its original character. Compared with the Empire and other veins, it is much more extensive, both laterally and vertically, and the values have gone deeper. In general it may be said that all the veins of the district...