SUMMARY. 185 



gards the ores and their manner of formation, the Leadville deposits of Col- 

 orado, described by Mr. Emmons, resemble those of Eureka most, but the 

 two regions differ widely in respect to the structure of the country and the 

 relations of the deposits to the different formations. A classification of the 

 ore deposits of this district, as regards their form, is a matter of considera- 

 ble difficulty. Some of them may be termed fissure or contact veins, but 

 in most cases they are very irregular in form and are better described by 

 the German word "Stock" than by any mining term used in English. 

 They are often lenticular, but this word cannot always be used to describe 

 them, as they often have offshoots in all directions. Any classification, 

 howe'ver, that is dependent on mere differences of form must be more or 

 less unsatisfactory. 



Distribution of the or e bodies.— The ore bodies do not seem to follow any partic- 

 ular direction either as regards dip or strike, and at first sight they appear 

 to be distributed throughout the ore-bearing formation without any regu- 

 larity. This is not wholly the case; and although no well-defined law can 

 be found governing their occurrence, this is connected with that of certain 

 phenomena in the country rock, such as fissures, caves, and broken lime- 

 stone. 



Formation of the ore bodies.— The distribution of the ore has been determined 

 almost entirely by the physical character of the limestone in which it is 

 found, and not by any chemical or mineralogical differences in the rock. 

 The greater facilities offered by a crushed and broken limestone, no matter 

 what its character, to the percolation of metal-bearing solutions would 

 more than compensate for any chemical advantages which a particular 

 kind of limestone might offer. During the process of upheaval to which 

 Prospect Mountain and Ruby Hill were subjected, the limestone was fis- 

 sured and crushed, great zones of shattered rock being formed, which are 

 separated here and there by unbroken belts. The ore-bearing solutions 

 entered the rock through the channels of least resistance, the crushed lime- 

 stone offering fewer obstructions than the fissures themselves, and deposition 

 followed in forms of a degree of irregularity corresponding to the complex- 

 ity of the preceding dynamic al effects. 



-Second Annual Report of the Director of the U. S. Geological Survey, 1881. 



