FOLDS AND FAULTS. 285 



It is rarely possible to trace upon the surface the actual line of a fault 

 or the structure lines of the immediately adjoining beds, for the reason that 

 the rocks are generally metamorphosed and disintegrated to such an extent 

 as to render them obscure. The theoretical studies of fault structure have, 

 moreover, been mainly made in underground workings, especially in coal 

 mines, where it is often the case that the movement of displacement is so 

 slight and the thickness of beds involved so small that it is questionable 

 whether they should not more projjerly be considered as joints, rather than 

 as fulfiUing the same conditions as these great faults many miles in length 

 and with displacements involving thicknesses of beds of as many thousand 

 feet. Even in this region, where the opportunities for observation are excep- 

 tionall}' favorable, the actual fault planes and the structure lines of the ad- 

 joining beds can but rarely be distinguished. Either only Archean rocks, 

 in which no structure lines are visible, are to be found on one side of the 

 fault, or the surface conditions are such that the structure lines are entirely 

 obscured in its vicinity. In drawing the sections, moreover, the endeavor 

 was to represent the facts as far as observed, without reference to any struct- 

 ural theory, and they were already engraved before any theoretical study 

 of the structure as a whole was undertaken. If, then, in any case they 

 misrepi'esent facts, the error is as likely to be against the above theory as 

 in its favor. 



At the northern edge of the map a syncline is plainly traceable in close 

 contact with the fault line on the west of the Mosquito fault, and the remains 

 of the corresponding anticline on its east side are found in the fragment of 

 Cambrian quartzite resting on the Archean just beyond the limits of the map. 

 From here southward to Empire gulch either the Archean alone adjoins the 

 fault line or the stratification lines of the sedimentary beds in its immediate 

 vicinity are entirely obscured ; tliose given in the sections are only the the- 

 oretical prolongation of dips observed at such a distance that there is room 

 for a very marked flexing to have occurred before they reached the fault 

 plane. On Empire Hill is what might be classed as a monocline to tlie west 

 of the fault, were it not that its continuation farther south at Weston's pass 

 shows that it is part of a deep syncline, cut off by the fault, and a portion of 



