ELY GREENSTONE. 155 



to do." Nor, showing volcanic characters as they do, can they be considered 

 as a part of the earth's crust produced by downward crystalHzation. They 

 must be, indeed, somewhat younger than the original crust. Nevertheless, 

 since the greenstones are the basement upon which rests a great series of 

 sediments that can be correlated with sediments in other areas which have 

 been regarded as of Lower Algonkian age, we have classed the igneous 

 greenstone basement in the Vermilion district as of Archean age. 



The intrusives that are considered to form a part of this complex 

 are those which are of essentially the same nature as the volcanics, but 

 which diifer slightly in their mode of occurrence. They are those portions 

 of the magma that penetrated the contemporaneous flows as dikes, and in 

 some cases, perhaps, are the material filling the conduits which connected 

 some of the flows with the magma mass from which they came. In one 

 instance a fragment of a greenstone was found included in a somewhat 

 different greenstone. The fragment was identified as being similar to, and 

 presumably derived from, one of the greenstones of the complex. The 

 intrusives included in the Archean greenstone complex belong to the same 

 period of formation as the lava flows with which they are associated. These 

 intrusive rocks are of essentially the same mineralogic and chemical com- 

 position as the volcanics themselves. 



It remains to be stated that we recognize the possibility, and, indeed, 

 the great probability, that there have been included in the areas mapped 

 as underlain by this Archean complex an occasional intrusive rock 

 considerably younger than the Ely greenstone proper. These greenstones 

 are cut by a number of dikes of relatively recent age and yet of essentially 

 the same character as the greenstones, except that they are less metamor- 

 phosed. No doubt many others were imrecognized, and, indeed, were 

 altogether unseen on account of poor exposures. 



COISTTACT METAMORPHISM OF ELY GREENSTONE. 



CONTACT EFFECT OF GRANITE ON THE ELY GREENSTONE. 



In the preceding pages general statements have been made concerning 

 changes which the greenstones have undergone since they were formed 

 In addition to those mentioned, which were essentially changes brought 

 about as the result of ordinary mountain-making forces and of percolating 



aThe origin of the Archean greenstones of Mizmesota: Geol. and Nat. Hist. Survey of Minnesota, 

 Twenty-third Ann. Rept., 1895, pp. 4-24. 



