606 CENTRAL MINERAL REGION OF TEXAS. 



to the siliceous nature of the Silurian dolomites which are traversed by the 

 veins, and it is also noticeable that lime and magnesia are much more abun- 

 dant than in the ores of the iron belts. Barium, which is a component of 

 most manganese ores, is so far as our analyses go wholly absent from those 

 of the Central Mineral Region. 



The great difficulty is to decide from what source of supply the manganese 

 minerals have been derived ; for it would seem that any process of solution 

 which might have drawn this material from the older iron ores would cer- 

 tainly have brought up the iron as readily, making the secondary product as 

 highly ferruginous as the original deposits. To some extent in certain situa- 

 tions this has been the case, and perhaps we may reasonably conclude that 

 the least ferruginous manganese ores have been formed under special modifi- 

 cations of similar conditions. Somewhat equivalent conditions in segrega- 

 tions may be observed in what appear to be more or less well defined belts 

 of the Cambrian hematites. That is to say, in some of the tracts in which 

 the iron segregations have been excessive the resulting ores have been highly 

 manganiferous, and this usually where there is a manifest distribution along 

 the northwest-southeast course in structural breaks. This feature is espe- 

 cially noticeable near the north end of Smoothing Iron Mountain, and on the 

 James River near the mouth of Salt Creek, as well as in extensions of both 

 these belts, the former across Llano County into San Saba and Blanco coun- 

 ties, the latter into Gillespie County, and to some extent in both directions in 

 Mason County. 



