MAGNETIC OBSERVATIONS. 337 



steadily gaining ground, that at least much of the iron of this magnetite was 

 originally buried in the same formations in which it now occurs, through the 

 agency of organic life. From this point of YJew the magnetite is in a cer- 

 tain sense a fossil, but with the important practical advantage over other 

 organic remains, that it need not be dug up in order to prove its existence. 



These magnetite-bearing rocks always produce disturbances in the 

 compass-needles held in their neighborhood. By a systematic location and 

 comparison of these disturbances the position of the rocks which produce 

 them can be determined Avitli a considerable degree of precision, even when 

 they are deeply buried. Besides their position on the map, the magnetic 

 observations may, and often do, indicate certain other geologically impor- 

 tant facts, such as whether the rocks are flat lying or highly tilted, the 

 direction of strike and dip, and, in some cases, the depth to which they are 

 buried. The methods employed in the field work were based on those 

 described by Maj. T. B. Brooks,^ who perfected the dial compass and pre- 

 dicted the importance of magnetic methods in geological mapping; but the 

 results reached in interprefation were gradually developed in the progress 

 of this work, as we were daily brought face to face with phenomena which 

 called for explanation. 



It must be clearly understood at the outset that in the iron ranges of 

 the south shore of Lake Superior magnetite is rarely concentrated in large 

 bodies, and that, in fact, its known occm-rence as such is restricted to a 

 small part of the western Marquette district, where in one ^Droducing mine 

 it now forms practically the whole product and in another a variable but 

 usually important part of the whole. It is therefore well understood in the 

 Upper Peninsula that disturbances of the magnetic needle, however great, 

 do not mean workable deposits of magnetite. Whatever significance such 

 disturbances possess is stratigraphical, and properly interpreted may lead to 

 discoveries of rich ore, other than magnetite, in formations to the position 

 and attitude of which the attractions may furnish a clew. But it may be 

 asserted as a general proposition, the essential truth of which has been 

 established by the experience of many years, that in the region referred to 

 magnetic distui'bances usually mean that magnetic iron ore in a workable 

 deposit does not exist in the area of disturbance. 



' Geological Survey of Michigan, Vol. I, Part I, 1873, Chapter VII. 

 MON XXXVI 22 



