CLASSIFICATION OF ORE DEPOSITS. 375 



he will be led by this very fact to make a more thorough and searching 

 examination than if he were only required to define the deposit in question 

 according to its outward form. 



As regards the applicability of the foregoing classifications to the Lead- 

 ville deposits, it will be seen from a perusal of the following pages that no 

 one of the subdivisions proposed would adequately define them ; either 

 they would apply only to a limited portion of the deposits or else they 

 would include them under the same head with deposits of an essentially 

 different character. 



Of von Cotta's, Lottner's, and Whitney's subdivisions, several would be 

 applicable; thus, a large part of the deposits are contact deposits; other 

 parts, however, not being at the contact of two different rocks, would be 

 stocks when large and pockets, chambers, etc., when small. The same remark 

 would apply to Pumpelly's subdivision of his Class II, 2. On the other 

 hand his definition of gash veins, as filling open fissures, would not apply 

 to those of this region. The deposits would come under only a single head 

 of Grimm's, Geikie's, and von Groddeck's classifications. By the two for- 

 mer the^y would be classed under the general head of stocks, which really 

 defines nothing except that they are of irregular shape and large. Finally, 

 von Groddeck's term "metamorphic," or "metasomatic," applies to all the 

 Leadville deposits and defines one most essential characteristic ; without 

 some modification, however, it would apply equally well to a large por- 

 tion of the Rocky Mountain deposits in Archean rocks, which have been pre- 

 viously considered to be " true fissure veins." 



LEADVILLE DEPOSITS. 



Manner of occurrence. By far the most important of the ores of Leadville 

 and vicinity, both in quantity and in quality, occur in the blue-gray dolo- 

 mitic limestone of the Lower Carboniferous formation, hence known as the 

 Blue or ore-bearing Limestone, and at or near its contact with the over- 

 lying sheet of porphyry, which is generally the White or Leadville Por- 

 phyry. They thus constitute a sort of contact sheet, whose upper surface, 

 being formed by the base of the porphyry sheet, is comparatively regular 

 and well defined, while the lower surface is ill-defined and irregular, there 

 being a gradual transition from ore into unaltered limestone, the former 



