abstracts: geology 471 



The manganese ore consists of the oxides, of which psilomelane, 

 pyrolusite, and manganite have been determined. Psilomelane is the 

 dominant oxide in all these deposits. It occurs in many forms but most 

 commonly in laminae deposited along the walls of fissures. Psilomelane 

 is more abundant in the surficial ores of the deposits; with increasing 

 depth the softer oxides, pyrolusite and manganite, become increasingly 

 abundant. Associated with the manganese oxides are calcite and sub- 

 ordinately, iron oxides. Calcite is universally present in the ores, though 

 it varies largely in quantity. Quartz was nowhere observed as a pri- 

 mary constituent of the manganese deposits, although the ore generally 

 carries a small quantity of silica from the inclusion of fragments of 

 siliceous wall rock. No manganese mineral other than oxides was 

 recognized in any of these deposits, but none of the workings had passed 

 through the oxidized zone. 



The manganese deposits of southeastern California examined in 

 this reconnaissance are similar in type and mineralogy to the deposits 

 in southwestern Arizona, but in each locality the source of the man- 

 ganese oxides is obscure. However, it seems probable that the man- 

 ganese oxides are the decomposition product of manganiferous calcite 

 deposited by rising hot solutions in the fissures, or that they were 

 deposited with calcite in the fissures by meteoric waters which obtained 

 the oxides from the decomposition of manganiferous minerals of the 

 surrounding rocks. The evidence for either hypothesis is not conclusive, 

 and definite proof can probably not be obtained until the deposits are 

 explored below the zone of oxidation. R. W. Stone. 



GEOLOGY. — Surface formations and agricultural conditions in the south 

 half of Minnesota. Frank Leverett and F. W. Sardeson. 

 With a chapter on climatic conditions by U. G. Purssell. Minne- 

 sota Geol. Survey Bull. 14: 8-148, map, pis., figs. July, 1919. 



This is the third of a series of three bulletins which treat of the sur- 

 face geology of Minnesota. The first (Bull. 12) deals with the north- 

 west quarter of the State, and the second (Bull. 13) with the northeast 

 quarter. Although published by the State of Minnesota, these bul- 

 letins represent results of a cooperative survey between the United 

 States and the Minnesota Geological Surveys. 



Each of the bulletins contains a discussion of the physical features 

 and climatic conditions of the entire State. In bulletin 14 a chapter 



