THE MINERALS OF IDAHO 91 



folded. Tracing the bands over to other faces of the cleavage cube, 

 they are seen to run parallel to the cleavage lines, not diagonal. 

 They are thus parallel to the plane (110). Bauer found in his experi- 

 ment that a motion or slipping was produced parallel to (110) and 

 this is clearly an instance of the same movement. Another struc- 

 ture, more sharply defined than the last, is caused by an alternation 

 of laminae with definite formal relations to each other. These are 

 sometimes thin, appearing as mere striations, but may have a thick- 

 ness of 2 centimeters or more. These laminae do not always occupy 

 the same position but are inserted parallel to several different crystal 

 planes. The law of twinning represented by many of these laminae 

 is: Twinning plane (331); axis, the normal of the twinning plane. 

 The structure of the galena showing this twinning is very complex 

 and has not been entirely worked out. 



Apparently several planes of twinning are present, all of which 

 lie in the zone 001 A 110, and including a number of different 

 trapezohedrons. The form (441) is mentioned as a twinning plane 

 for galena by Sadebeck, and (131) by Zepharovich. In small frag- 

 ments twinning laminae occur parallel to the octahedron (111). By 

 careful examination of rnaivy of these specimens striations are seen 

 running at regular angles with the lines of the cubic cleavage. By 

 measuring these inclinations under the microscope, the faces parallel 

 to which the laminae causing the striations are inserted may be 

 determined. This twinned structure is certainly secondary and 

 probably due to pressure. In the specimens the evidence of pressure 

 is very marked, and the distorted forms of the masses which are 

 included by the cleavage planes are sometimes very striking. These 

 should be cubes but are in fact rhombs or bodies with unsymmetrical 

 shape. Faces which are not regularly disturbed by the slipping 

 parallel to (110) are often curved and show indistinct development 

 of one or the other of the twinning forms. It seems that the pressure 

 manifested itself in accordance with the position in which a given 

 individual lay with regard to the pressure. Sometimes the force 

 could be all applied to the production of one form of twinning; some- 

 times it resolved into several elements of pressure, each availing 

 itself of the plane of weakness most suitable for its manifestation 

 and hence different twinnings and slippings are developed all in 

 one piece. In many places, probably in the greater part of the mass 

 of the ore, the galena has been simply crushed, as is illustrated by 

 specimens with slickensided surfaces. Again a very thin and fine 

 lamination has been produced, combined with much fracturing, so 

 that the law of the structure has been obscured. 61 



41 Whitman Cross. Proc. Colorado Sci. Soc, vol. 2, p. 171, 1885. The discussion by Cross, reprinted 

 here with slight changes in wording was the first accurate and lucid discussion of the crystallographic proc- 

 esses leading to the production of the banded and gneissic twinned structure in galena which is common 

 in Idaho and elsewhere and which lias since been described by several other writers. 



