VERMONT ASBESTOS 435 



occupied a greater breadth or area than the zone of thrust. Nowhere 

 in the field are small slip-blocks better developed than at the Tucker 

 contact. The accompanying illustration (plate 75, figure 1) gives some 

 idea of the form and character of the surface. It will be seen that the 

 surface has been polished and sometimes covered with a fibrous film of 

 hornblende, which has been stretcbed or pulled out in the direction of 

 movement. In consequence of the shearing the trituration of the blocks 

 upon each other has produced alternate sharp angles around the greatest 

 periphery, with alternating obtuse edges roughly in a plane at right 

 angles to the plane of the former and along its greatest diameter, or as 

 many as three obtuse angles may occur on each flat surface of the block. 

 In cross-section minute partings may be seen lying roughly parallel to 

 the trend of the surfaces and filled with glistening films of fiber — that 

 is to say, the strain to which these masses were subjected was so great 

 that they in turn were also fractured aud subsequently knitted together 

 by the deposition of silky fiber. These facts are shown in the sketch 

 (figure 3) of the cross-section of one of these blocks. 



STRUCTURE OF CROSS-FIBER VEINS 



A cursory examination of any block containing typical cross-fiber 

 veins shows that they usually occur in groups of bifurcating and rejoin- 

 ing members, usually associated with one or two larger members not 

 exceeding 1 inch in diameter. This feature is shown in the following 

 sketch. 



In thin-section of even the microscopic veins magnetite and probably 

 a small amount of chromite are found to be constant associates. Pyrite 

 was also recognized, but in very small amount. In the hand specimen 

 it would appear that the magnetite was confined very largely to the cen- 

 tral portion of the individual vein. In thin-sections of the smaller veins 

 this relationship is not so evident. On the contrary, sections may be 

 selected in which there seems to be no localization of the metallic con- 

 tents, which are scattered throughout the entire width of the fiber band 

 and extend well into the body of the serpentine. This, however, is not 

 the rule, but rather the exception. From the material at hand I think 

 we may safely conclude that the ores have a decided tendency to ac- 

 cumulate within the central zone of the fractures, while the deposition 

 of the fiber favors the walls. 



DISCUSSION OF PROBABLE MODES OF GROWTH 



The method of growth of the fiber and its relation to the ores is a vexed 

 problem. Whether its deposition begins on the walls and gradually 

 pushes its growth toward the central space and finally coalesces along a 



LIX— Bull. Geol. Soc. Am., Vol. 16, 1904 



