LEAD. 589 



locus of the galena was the limestone itself or the substratum through which 

 the granite was protruded. Still another difficulty arises from the absence of 

 fossils in the containing rocks. We have therefore only the knowledge which 

 comes from a certain familiarity with the Cambro-Silurian section, its practical 

 identity in the separated lead bearing areas, and a not very narrow range of 

 strata lying between the known Cambrian and known Silurian, as adjudged 

 hy fossil evidence. 



In every instance thus far discovered the galena occurs in rocks not older 

 than the Potsdam greensand and at least as old as the upper part of the Leon 

 series of the Silurian. A certain greensand, probably Potsdam, but possibly 

 Silurian, is. a constant accompaniment, and as before stated an intrusive gran- 

 ite of Silurian or Post-Silurian age invariably appears in contact with the Cam- 

 brian beds, or with beds low in the Silurian system. But there are usually 

 faults in the vicinity which bring the Silurian and Cambrian beds into such 

 close proximity that new doubt is introduced as to the local origin of the 

 galena deposits. In the Beaver Creek district (Fig. 65) and the Pedernales 

 Valley tract (Fig. 66) the relations of the Post- Silurian faults to the mineral 

 accumulations are very evident, and the same' is true of the Mason County 

 area (Fig. 67), except that there the structure is so chaotic as to preclude any 

 attempt at correlation in detail with other districts. 



We may, perhaps, get a further clew to the history of ore deposition from 

 this last named region; for while the Silurian trend is prominent here as in the 

 other fields, the later uplifts have apparently disjointed and separated the lead 

 deposits instead of forming new channels for their accumulation. This fact 

 may perhaps imply deposition earlier than the Devonian Period. 



The Cavern limestone of the Silurian Period and the galena bearing green- 

 sand lie in juxtaposition in the fault fissures on the south side of Silver Mine 

 Hollow, Burnet County ; a similar slip in the Blanco County tract has brought 

 the same greensand to the level of the Hoover strata along a parallel fault; 

 but at Caylor's there is hardly a possibility of determining accurately what 

 conditions prevailed immediately after the Post-Silurian disturbance. More- 

 over, we have no evidence that the Devonian or Carboniferous strata ever 

 formed a cap in these regions. The presumptions based upon known facts 

 are against such a belief. The chances are that the processes of vein forma- 

 tion of the ordinary kind have never since been as favorably conditioned in 

 these areas as when the initial effects of the Silurian upheaval were active. 



From thoughtful consideration of all the known facts I can not reach any 

 other conclusion than that the galena is a product of about the same age and 

 character as the lead masses of Missouri, formed originally under very simi- 

 lar circumstances; but that our ore deposits are less extensive, and have been 

 much more affected by disturbances of the strata occurring since the epoch 



