580 CENTRAL MINERAL REGION OF TEXAS. 



the rock. Dykes or bosses of white quartz mark the axial line of this belt, 

 as well as others in the Burnetian area, but there have been no reports of any 

 occurrence of rare metals in such masses in the copper fields. Much of the 

 quartz is barren, but in some places it forms the matrix for the silver bearing 

 gray copper ores, which are sometimes associated more or less with the cop- 

 per sulphides — chalcopyrite, chalcocite, and occasionally bornite. This is the 

 condition in the "Mexican Diggings," near Babyhead Mountain, and in the 

 westward continuation of the belt on Babyhead, Wolf, and Pecan creeks, and 

 in the patches exposed in the upper valley of Cold Creek, all in Llano County. 



Another small outcrop directly in the course of the same axis has recently 

 been worked with encouraging results by J. M. Boroughs, of Austin, near 

 the head of a branch of Deep Creek, in San Saba County. The rock taken 

 from this locality is thus far gneissic, highly impregnated with malachite and 

 carrying thin seams of gray copper (tetahedrite). This class of ore and its 

 mode of occurrence here described are especially characteristic of the north- 

 ern edge of the uncovered Burnetian area. The belt is less than six miles in 

 width. It includes the whole of the Babyhead district and the Wolf -Pecan 

 Creek district. Eastward and westward from that region it is mostly ob- 

 scured by Cambrian or Silurian deposits. The Boroughs diggings are in the 

 most northern locality known to be exposed. A line drawn through this 

 spot, however, in the direction of the Burnetian axis, passes through several 

 points where gneissic rocks outcrop, and where there are structural evidences 

 of unconformity of deposition in the sedimentary beds. This line crosses 

 Hinton Creek and Deer Creek at places which exhibit peculiar contacts, im- 

 plying that some ancient terrane exerted an influence upon deposition, even 

 as late as Silurian time. The eastward prolongation of the line traverses an 

 area wholly covered by Cambrian and Silurian strata until it reaches the 

 head of Little Llano Creek. Thence it passes along the edge of the Babyhead 

 copper-silver tract, and then crosses another area covered by Cambrian and 

 Silurian rocks. Farther eastward come in the granitic rocks of the Colorado 

 River, near the mouth of Beaver Creek, and those exposed on Beaver Creek 

 and Silver Mine Hollow. These seem to be more intimately connected with 

 later uplifts, but it is at least possible that they may have derived their pres- 

 ent position in part from some weakness along the old Burnetian axis. 



The copper deposits of other parts of the Central Mineral Region do not 

 belong to the Northern Belt, although they lie in parallel folds. 



THE MIDDLE OK MASON BELT. 



South of the Llano-San Saba tract there is an area in which the gneisses 

 and schists of the Burnetian System are well exposed in the eastern half of 

 Mason County, where they are copper bearing in places. As in the north- 



