134 THE VERMILION IRON-BEARING DISTRICT. 



anticlinal tongues separated by sedimentary formations, and, after varying 

 materially in width as the result of cross folding, finally ends at its eastern 

 extremity also in two tongues — eastward-plunging anticlines partially 

 surrounded by the younger sediments. 



EXPOSURES. 



The exposures of greenstone in the areas outlined above are very good. 

 It is no uncommon thing to find almost absolutely bare surfaces several 

 hundred feet long and possiblj^ one-fourth as wide. Such exposures are 

 most commonly rounded surfaces. Occasionally, however, cliffs of green- 

 stone are seen. In spite of the large size and the great number of the 

 exposures, it was very difficult — in fact, in most places almost impossible — 

 to determine the relations of the different kinds of greenstone to one another, 

 for the contacts have usually been concealed either by drift or by the 

 effects of erosion, so that where most needed, as is commonly the case, the 

 exposures are wanting. Mention will be made later of a few places where 

 some of the best exposures were found. 



TOPOGRAPHY. 



In the western jjortion of the district, where the the greenstone under- 

 lies broad areas, the topography is very much broken. The minor idges 

 are numerous and form the most prominent feature. In this portion of the 

 district there is a series of parallel ridges with narrow valleys between 

 then^. Usually the sides have a steep slope, and there are sometimes abrupt 

 escarpments, but as a rule the hills and ridges are well rounded. It will 

 thus be seen that in detail the topography is very rugged. Especially is 

 this so north of Fall and Long lakes. The ridges throughout the green- 

 stone area lie in essentially parallel chains extending east-northeast, a 

 direction corresponding to the trend of the structure of the district. It has 

 already been stated that the Ely greenstone occurs very rarely in broad 

 areas in the eastern portion of the district, being there usually found in 

 comjDaratively small areas surrounded by younger sediments. There is a 

 very noticeable difference in the topography of areas underlain b}' the 

 greenstone and those underlain by the surrounding sediments, due to 

 differential erosion. As a rule, the greenstone forms the prominent hills 

 and main ridges. Usually, in traversing' the country, one finds that after 

 leaving the sediments, which lie within a topographic depression, there 



