TERRANES. Ill 



mountain-summits. The faulted region, because a region of fractures, is, in 

 general, the course of a great valley. 



Pissures also are faulted in the several ways mentioned, because they 

 are in the terranes, and must share in the displacements. They may be 

 faulted even at the time when they are first made, and faulted at various 

 later periods. 



Besides faults of up-and-down displacement, there are also (1) longitu- 

 dinal faults, and sometimes without much change of level in the beds. 

 More common than either vertical or longitudinal faults are (2) the oblique, 

 since resistance and pressure would seldom be so equable as to prevent 

 obliquity. (3) Horizontal displacement of strata also occur, and sometimes 

 of marvelous extent. They are produced by a horizontal or oblique thrust 

 shoving terranes over others. In a case reported from the Scottish High- 

 lands, a mass of the oldest crystalline rocks, many miles in length from 

 north to south, was thrust at least ten miles westward over younger rocks, 

 part of the latter fossiliferous. 



(4) Bed-plane faults are still another kind in which the plane of displace- 

 ment is that between two layers or strata. They are produced by the push- 

 ing of one bed or stratum of a series over the surface of that below it. In 

 the Triassic of East Haven, Conn, (on the borders of New Haven), the 

 successive beds of the red granitic sandstone (which dip eastward 16° to 20°) 

 have been shoved over one another upward along the plane of bedding, 

 producing large and general displacements, and great slickensided surfaces; 

 and these surfaces have generally a very thin and hard white coating, -ap- 

 parently due to the ground-up feldspar. In the same region, besides these 

 shoves of layers over one another, there are also ordinary faults with slick- 

 ensided walls ; and in many places the rock is in fragments, and all the 

 fragments, even those no larger than the hand, indicate participation in the 

 movement by the slickensides which cover them. 



(5) Pressure has sometimes produced a crushing of the rocks along frac- 

 tures, either directly or aided by lateral movement, making what has been 

 called in the latter case shear-zones. 



(6) In the upturning and flexing there has also been slipping, by the 

 inch and fractions of an inch, along planes of cleavage or bedding, making 

 slip-faults, and producing also small flexings or crumplings of the beds. 



Jointed structure., joints. — A jointed structure is a style of fracturing, 

 usually on an extended scale, in which there is a degree of system in the 

 arrangement of the fractures. The divisional planes are termed joints. 

 They cut across the stratification, and may have great extent vertically and 

 laterally. The planes of division are often very even, and not enough open 

 to admit the thinnest paper. They may be in one, two, or more directions 

 in the same rock, and extend, with nearly uniform courses, through regions 

 that are many miles in length or breadth. The accompanying sketch repre- 

 sents the falling cliffs of Cayuga Lake, and the fortress-shapes and buttresses 

 arising from the natural joints intersecting the rocks. The wear of the 



