288 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



only one fault crosses the section, the linear contraction of a given bed, as 

 there drawn, is about three thousand feet in the length of the section, or 

 3i per cent. On Section D the apparent amount of contraction is the same, 

 although the beds are much more sharply flexed ; but it is found that, by 

 reason of the angle of hade given to the faults, there has been 1,500 feet 

 of apparent expansion of the beds; or, if the fault planes had been made 

 vertical, the same amount of flexing would have given 1,500 feet more 

 length to the beds and the contraction would have been 4,500 feet, or 5J 

 per cent. The sections present probably an exaggerated statement of what 

 actually exists, for it is possible and even probable that the planes of the 

 great faults stand more nearly in a vertical position ; still, observation ren- 

 ders it probable that the average hade in the faults of this range is with 

 the downthrow, and for this reason the displacement of the faults has not 

 tended to contract the linear distance occupied by a given series of forma- 

 tions on a transverse line, but rather to expand it slightly. It seems proba- 

 ble that the plication of the beds has been a gradual and uniform move- 

 ment, though relatively accelerated at the period assigned to the dynamic 

 movements; but that the actual fracturing of the beds along the present 

 fault planes was primarily produced by some violent shock, similar to the 

 earthquake shocks of the present day; that the direction of a fracture 

 plane across the beds, as thus primarily determined, would not necessarily 

 be dependent on the force of contraction, although its position would 

 naturally be on lines of greatest tension or weakness. 



It may also be conceived in a region like the one under consideration 

 that, while the folding is evidently a result of tangential contraction, the 

 faulting may be, in part at least, the result of radial contraction. It is 

 probable that tangential pressure acts only on a comparatively thin shell of 

 the upper crust of the earth, for very sharp folds, where observation in 

 depth is possible, are found to become gradually more rounded and gentle 

 as the distance from the surface increases ; also, that the force which has 

 been exerted in an intensely plicated region is the expression of the accu- 

 mulated energy of contraction over a wide area. Thus, in the case of the 

 Mosquito range, tangential pressure may be conceived to have pushed up a 

 roll of the earth's crust into a ridge, which would have been much higher 



