16 KEMP'S ORE DEPOSITS. 



igneous rocks, have often so broken up a district that it is a matter 

 of much difficulty to trace out the course through which it has 

 passed. Subsequent erosion, or the superposition of heavy beds of 

 gravel or forest growths, etc., may so obstruct observation even of 

 the facts as to add to the obscurity. The expense of making and the 

 consequent scarcity of accurate contour maps to assist in such 

 work are other obstacles. The profound dynamic effects wrought 

 by mountain-making processes, although in individual cases pro- 

 ducing only the simpler phenomena already cited, yet in general 

 are much more extensive, and must be considered in the study of 

 many large districts. When folds are the result of compression or 

 thrust, the dynamic effects are more marked than in those formed 

 by sagging. Faults are larger and more abundant. When sedi- 

 mentary beds have been laid down along an older axis of granite 

 or some equally resistant rock and the thrust crowds the beds 

 against this axis, the conditions are eminently favorable to great 

 fracturing and disturbance. The flanks of the Rocky Mountains 

 furnish such examples. 



1.02.05. There are also great lines of weakness in the outer 

 portion of the earth, which seems to have been the scene of fault- 

 ing movements from a very early period. Thus on the western 

 front of the Wasatch Mountains, in Utah, is a great line of weak- 

 ness, that was first faulted, as nearly as we can discover, in 

 Archaean times, and has suffered disturbances even down to the 

 present. A few instances of actual movements within recent 

 years have been recorded. In 1889 a sudden small fold and fissure 

 developed under a paper mill near Appleton, Wis., and heaved the 

 building four and a half inches. (See F. Cramer, "Recent Rock 

 Flexure," Amer. Jour. Sci., III.,.xxxix. 220.) This occurred in 

 what was regarded a settled region and one not liable to disturb- 

 ance. 



1.02.06. Wherever igneous rocks form relatively large por- 

 tions of the globe they necessarily share extensively in terrestrial 

 disturbances. Not being often in sufficiently thin sheets, they 

 rarely furnish the phenomena of dip and strike. Folds are largely 

 wanting. They are replaced by faults and shattering. The fis- 

 sures thus formed are at times of great size and indicate impor- 

 tant movements. The Comstock Lode fissure is four miles long 

 and in the central part exhibits a vertical displacement of three 

 thousand feet. (See 2. 1 1. ] 9.) Such fissures seldom occur alone, but 

 minor ones are found on each side and parallel with the main one. 



