A. F. Rogers — Manganese Minerals. 449 



in chalcedony and quartz. It is probably of Franciscan 

 age, as are all the other manganese deposits of the Coast 

 Eanges. The occurrence of hausmannite at a mine in 

 San Luis Obispo County 4 and an undescribed occurrence 

 of tephroite near Mount Diablo both in rocks of Francis- 

 can age gives credence to this belief. Franciscan cherts 

 and jaspers outcrop two to three miles east of the spot 

 in which the bowlder is located. Several manganese de- 

 posits are found here but as far as examined they are 

 different miner alogically from the so-called meteorite. 



The typical occurrence of tephroite and hausmannite 

 is in contact-metamorphic deposits. No limestone is 

 present in the vicinity of Alum Rock Canyon and none 

 was found in the bowlder. Limestones of Franciscan 

 age are known at various places in the Coast Ranges but 

 they are unmetamorphosed. 



Although some doubt is attached to the original occur- 

 rence of this manganese ore, the presence of tephroite 

 and hausmannite points to the high-temperature nature 

 of the deposit. The rhodochrosite is undoubtedly a 

 hydrothermal mineral and the psilomelane doubtless was 

 formed by meteoric waters at a very late period. The 

 pyrochroite is intermediate in age between the rhodo- 

 chrosite and the psilomelane. Was it formed by meteoric 

 waters or by hydrothermal solutions! This question 

 cannot be answered definitely, but it is believed to have 

 been formed at a late hydrothermal stage. It is altered 

 to a later hydrous manganese oxide and so is unstable 

 under oxidizing conditions. In fact specimens alter 

 when exposed for several days. The presence of a large 

 amount of water [Mn(OH) 2 ; H 2 O = 20-3 per cent] is no 

 proof that a mineral is formed by meteoric waters. 



Such is believed to be the essential history of this 

 interesting deposit or rather remnant of a deposit, 

 though a number of questions can not be settled for lack 

 of sufficient data. The existence of this bowlder makes 

 it probable that some, if not many, of the manganese 

 deposits of the Coast Ranges of California are of high 

 temperature hydrothermal origin. The prevalence of 

 psilomelane may be accounted for by subsequent altera- 

 tion of the hydrothermal minerals by meteoric water. 



Stanford University, California, 

 September, 1919. 



4 Bull. 76, California State Mining Bureau, p. 14, 1918. 



