604 CENTRAL MINERAL REGION OF TEXAS. 



are so persistent and so distinct from the strictly Fernandian aspect of the 

 principal iron fields that a different mode of origin is plainly indicated. 



In my Report for 1889, page 307, allusion was made to a series of structural 

 breaks in Silurian strata corresponding in trend to the buried Fernandian 

 axes. This is not the place to discuss such topics in detail, but it may be re- 

 marked that more extended observations have made it appear very probable 

 that the manganese deposits are really secondary aqueous vein formations of 

 Post-Silurian date. They follow the northwest trend frequently, but other ac- 

 cumulations lie in the trend (north 25° east) of the uplift at the close of the 

 Silurian Period. These two vein courses occur together in the same fields, 

 and almost invariably there is evidence of a later origin than the adjacent 

 rocks whenever the manganese ores are exposed. 



It should be understood that the bands drawn across the Economic Map 

 are intended to indicate the courses of the outcrops, and not to imply the oc- 

 currence of manganese ores continuously. There will be no use prospecting 

 over the areas in which the Burnetian rocks are at the surface. 



THE SPILLER BELT. 



The Spiller mine has been left practically in the same condition reported 

 last year. Such prospecting as has been done in adjacent areas has vindi- 

 cated the statement in my former Report (page 346) to the effect that " pros- 

 pecting in the locality would be justifiable from the surface indications." 



The chief element of uncertainty is the vein-like character of the ore bodies, 

 which are liable to prove irregular and pockety as well as uneven in quality. 

 The showing of the Spiller mine is, however, very encouraging, and there is 

 good reason to look for a continuance of that deposit in a southeastward 

 course. Little can be predicted as yet of the companion course (north 25° 

 east), although what evidence we have is less favorable for continuous veins. 

 In a northwestward direction the chances for outcrops of the ore are not as 

 good, but there are some localities where it would not be surprising to dis- 

 cover similar deposits These are indicated in a general way upon the ac- 

 companying Economic Map (Plate XXII). 



The mineral is principally psilomelane or a near ally, but there is much 

 variety in composition in different parts of the belts. Some samples approach 

 more closely to pyrolusite, and some even to braunite, while much of it is so 

 irregular a mixture as to be more properly regarded as "wad." Silica in 

 excess is characteristic of a considerable portion of the product from the 

 northern portion of the belt near the Spiller mine; farther west (Pluennecke 

 survey, No. 764, Mason County) the material can only by courtesy be re- 

 garded as a manganese ore. This last is really a manganiferous iron ore 



