| OCR Text |
Show DIFFERENT EPOCHS OF DISPLACEMENT. 43 eruptive district seem to have had portions of their shearing before the beginning of the principal epoch of displacement. But these earlier symptoms are usually like old wounds which had once healed and afterwards broke out again with increased disorder. The Sevier fault, in particular, shows signs of two epochs of activity in some portions of its extent. Between Monroe and Gunnison it appears as a fault cutting along the axis of a small but sharp monoclinal flexure. The flexure is clearly older than the fault. The Musinia faults cut obliquely across the great monoclinal of the Wasatch Plateau, and show little sympathy with it. The Paunsagunt fault, uniting with the northern extension of the East Kaibab flexure, is plainly independent of it, and is decidedly younger.- It is a most curious circumstance that where we find this two-period displacement the motion of the fault is often reversedâ€"the lift of the first period is the throw of the second. It is not always so, but I believe it to be true in a majority of cases where the double movement has been detected. On the other hand, where the shearing of both periods has been in the same direction the movements would be much more difficult to separate, and many such double movements doubtless have escaped observation. All of the displacements thus far discussed belong to the same system. Whether older or younger, they lie along the same lines and very seldom show any interferences. None of them will go back of the Pliocene in ao-e and I think it probable that none of them will go behind the middle Pliocene. Older displacements along these lines, if they exist, are wholly covered up and obliterated, and cannot be separated at present from the later ones of this system. There is, however, a totally distinct system of displacements, belonging to a much earlier age, which the grander and more general erosion of the country has brought to light, but which can never be confounded with the Pliocene-Quaternary system. They make a wide angle with the latter series and have a history wholly independent of them. They are only occasionally revealed in a fragmentary manner in places where deep gorges have cut through thousands of feet of Tertiary formations and volcanic emanations, or where erosion has swept off corresponding amounts of strata from broad districts. Only in two or three places in the heart of the High |
