MINERALOGY AND PETROLOGY. 999 



Note.] 



and banded in the manner of rhyolyte 'and not that which is interbedded with 

 greenstone debris, or which passes into argillyte or other rock by slow changes. 



5. Although these volcanoes may have begun as submarine, the surrounding 

 areas must have been elevated gradually above the ocean level. Their lavas then 

 would have remained largely unsilicified, and they may have flowed great distances. 



6. Why are some lavas silicified and others not? Without going into this 

 question any further, it may be well to suggest some causes for this difference: (a) 

 Some flows may have been submarine and some terrestrial, (b) Some may have come 

 from volcanoes, at first submarine, and some from fissures that never formed volcanoes. 



(c) Some lava layers may have been very thick too thick for complete oceanic 

 silicification.* In that way large bodies of "red rock" may have been formed. 



(d) The great dynamic fracture line may have been shifted, as stated in the preface 

 of volume iv (page 17), to the south. Indeed, the Beaver Bay diabase and all later 

 (Manitou) lavas may have been ejected from such later fissures. 



7. The ore bodies are likely to be found in the vicinity of the ancient craters, 

 where oceanic precipitation was most copious, and hence in varying amounts on the 

 same stratigraphic horizon. 



8. Although to a small extent the process of alteration may have continued to 

 the present, the bulk of the iron, as of the quartz, of the iron-bearing rock, must have 

 originated during the period of volcanic activity. 



Note. Circumstances render it impossible to thoroughly digest the new issues 

 involved in the adoption of this hypothesis. That it bears- an intimate relation to 

 the Animikie, the disappearance of which throughout a long tract in Minnesota has 

 caused much difficulty of interpretation, and to the whole "red rock" series, 

 especially to the more massive parts of the " red rock," is at once apparent. It brings 

 the flinty layers of the Animikie, along the northern strike of the Animikie, into 

 relation with eruptive causes, and suggests that the Cabotian age of igneous activity 

 may have been (or at least begun) in early Animikie time; and that involves the 

 definition of the Keweenawan. 



*The silicification of trees, a common feature of some parts of the western United States, especially in Arizona, seems to 

 be a similar phenomenon. 



