198 THE VERMILION IRON-BEARING DISTRICT. 



of volcanic activity, or some old flows which have poured out from the land 

 into the adjacent sea while the iron-bearing formation was being deposited. 

 In no case, however, has a jasper mass been found which could be conclu- 

 sively shown to be included in the greenstone as a result of igneous intrusion. 



RESUME OF RELATIONS TO ELY GREENSTONES. 



In view of the above-described modes of occurrence of the iron 

 formation in association with the greenstone, we reach the following 

 conclusions concerning the age relations of the two : A portion of the iron 

 formation — and this appears to form by far the predominant part of it — is 

 clearly younger than the greenstone; for instance, those masses of the 

 formation that overlie the clastic sediments which have plainly been derived 

 from the underlying greenstone. Other very subordinate portions of the 

 formation are interbedded with the greenstone, and hence are partly 

 contemporaneous with it. This is shown in those cases where there is 

 greenstone overlying- the iron formation. Moreover, it is not improbable, 

 although not susceptible of definite proof, that some of the smaller areas of 

 the iron formation are included in a greenstone which has been later 

 intruded through the iron formation. Lastly, small areas of jasper, chert, 

 and ore, similar in g-eneral characters to the iron-bearing rocks, are second- 

 ary infiltration products. 



In order to get a clear understanding of the conditions which would 

 permit such a variety of relationships between the greenstones and the iron 

 formation, it is necessary that we call to mind the conditions under which 

 these two formations originated. A study of the greenstones has led to 

 the conclusion that they were formed by volcanic outbursts. Just as at 

 the present day we have lavas and tuff masses outpoured upon the land 

 and partly occupying adjacent water areas, sedimentary deposits being 

 formed offshore where the conditions are favorable for them, just so did 

 we have similar conditions in the early history of the Vermilion district. 

 As a consequence of this volcanic outburst on the land and the simultaneous 

 formation of sedimentary deposits in the sea, we now find the two 

 intermingled. In this waj^ we can conceive that clastic sedimentary 

 deposits might be derived from and overlie lava flows, and grade up into 

 nonclastic sediments. Where conditions were not favorable for the 



