K. V. Shannon — Occurrence of Ilvaite. 119 



occurs is of contact-metamorphic origin. 1 The data on 

 the South Mountain occurrence are incomplete but in the 

 opinion of at least two competent observers, the minerals 

 of the ore in which the ilvaite occurs are of contact-meta- 

 morphic origin. If this is true, this must be one of the 

 few known deposits, of true contact origin, which contain 

 lead as the predominant metal. The amount of silver 

 present is also unusually high for a contact deposit. 



If the diagnostic value of the mineral is to be conclu- 

 sively proven, it is important that the occurrences of the 

 mineral be fully described. Most of the localities, thus 

 far discovered, have been described in languages other 

 than English and it is possible that some of the papers 

 already published prove the mineral to have originated 

 by other processes. Ilvaite is of interest to the mineral- 

 ogist especially because each of several authorities who 

 have worked upon the crystallography of the mineral has 

 given a different axial ratio. The specimens described 

 below are interesting because of the large size of the 

 crystals, these being perhaps the largest ever found in 

 the United States. The results of the crystallographic 

 work have not been such as to settle any of the doubtful 

 points regarding the mineral. It is possible, however, 

 that, were a special search made with a view of obtaining 

 smaller crystals, more suitable for crystallographic 

 investigation, this locality might furnish material on 

 which the crystallographic constants of the mineral could 

 be accurately determined. 



The ilvaite from South Mountain was first identified 

 by Dr. W. F. Hillebrand and Prof. F. W. Clarke in speci- 

 mens of ore from the Golconda Claim, collected by Dr. 

 Waldemar Lindgren. The ilvaite is mentioned by Lind- 

 gren in the brief description of the deposit in which it 

 occurs. 2 Later an analysis of the mineral was published 

 by Hillebrand. 3 Aside from these incomplete references, 

 the only mention of the occurrence is to be found in the 

 report of Bell, for 1906. 4 The locality is not mentioned 

 in any of the standard works on mineralogy. 



Location and geology of South Mountain. — South 

 Mountain is located some 20 miles southwest of Silver 



1 Irving, John Duer, Economic Geology, Lecture Course, S. S. S., Yale 

 University, 1916. 



'Lindgren, W., Gold and Silver Veins of Idaho, 20th Ann. U. S. G. S., 

 Pt. Ill, 1900. 



a Anal, of Bocks and Minerals, U. S. G. S. Bull. 591, p. 318. 



••Bell, Robert M., 8th Ann. Kept. Mining Industry of Ida., Boise, 1906. 



