A. F. Rogers — Manganese Minerals. 443 



Art. XXXI. — An Interesting Occurrence of Manganese 

 Minerals near San Jose, California; by Austin F. 

 Rogers. 



For some years a huge bowlder of manganese ore situ- 

 ated on the bank of Penitencia Creek below Alum Rock 

 Park which is about five miles east of the city of San 

 Jose, Santa Clara County, California, bore a placard 

 which proclaimed it to be a meteorite. All geologists 

 familiar with it knew that the so-called meteorite was 

 simply manganese ore, for it showed a dull black mineral 

 recognized to be psilomelane, the common manganese 

 mineral of the Franciscan formation of the Coast Ranges 

 of California. In the autumn of 1918 when restricted 

 shipping facilities created an urgent demand for domes- 

 tic manganese ores, the city of San Jose, owner of the 

 land on which the bowlder was located, at the request of 

 the government decided to dispose of it. Several hun- 

 dred tons of high-grade manganese ore* was obtained 

 from it and shipped to a California steel plant. 



My attention was called to the manganese ore of a 

 "Aluni Rock meteorite," as it was known locally, by a 

 resident of San Jose who presented me with a small 

 specimen from its interior. Great was my surprise 

 to find that this specimen was largely pyrochroite, 

 Mn(OH) 2 , and not psilomelane. As soon as opportunity 

 was afforded, a visit to the locality was made and a suite 

 of specimens obtained. The minerals identified in the 

 bowlder are : Tephroite, hausmannite, pyrochroite, gano- 

 phyllite, rhodochrosite, barite and psilomelane. Of these 

 the first four are very rare, except at Langban, Sweden, 

 and ganophyllite is known only from this locality. Haus- 

 mannite is not certainly known in the United States 

 but has been reported from San Luis Obispo County, 

 California. Pyrochroite occurs at Franklin Furnace, 

 New Jersey. Tephroite in this country has been found 

 in but two localities, Franklin Furnace and central Texas. 



After the minerals are described, an attempt will be 

 made to determine the relations of the minerals to each 

 other and the character of the deposit. The facts 

 brought out in this discussion may have a bearing on cer- 

 tain geological problems pertaining to the Coast Ranges 

 of California. 



* The ore shipped varied in manganese content from 43-2 to 63-5 per 

 cent and averaged 52-6 per cent, according to Dr. W. C. Bailey, City 

 Manager of San Jose, to whom I am indebted for this information. 



