328 N. H. WIXCIIELL OJUGl^s OF LAKE SUPERIOR IRON ORES 



seen in the opposite margins. The massive eontorted jaspilyte is that 

 which was formed by siliciiieation of a mass of basic lava suddenly cooled 

 by contact with the oceanic water, its tiuidal structure preserved. The 

 stratified noncontorted jaspylite is that which was formed by chemical 

 precipitation consequent on the disturbance caused by the extrusion of 

 the igneous .rock now represented by the contorted jaspilyte. A further 

 inspection shows that the contorted mass itself is composed of two parts, 

 both of extrusive origin, one nonconformable on the other. That the 

 stratified jaspilyte was of sedimentary origin is shown by its grading 

 conformably into other sedimentary strata, ' such as green schist, slaty 

 graywacke, and finally into coarser grit and graywacke. 



But there is a third kind of jaspilyte, which sometimes becomes al- 

 most merchantable iron ore. It is also of detrital sedimentary origin, 

 and in the vicinity of the otl:er two kinds the individual detrital pieces 

 may be very coarse, even large masses of jaspilyte of the contorted kind, 

 as at the west end of the Lee hill at Tower, so sparsely mingled with 

 other sediment that they may appear indigenous. This third kind of 

 jaspilyte also has been the cause of perplexity to the field observer, but 

 on careful examination over a wider area it has been found to graduate 

 into finer and finer detritus and to pass into some of the stratified gray- 

 wackes. It is needless to remark that this third variety of jaspilyte de- 

 notes a nonconformity in the rocks of the region, and that it belongs 

 above the plane of nonconformity. At Tower, in the Lee hill, it is a 

 part of the basal conglomerate of the Tapper Keewatin and a phase of the 

 Ogishke conglomerate, the great mines being all in tlie T^ower Keewatin. 



The same features and many of tliese jaspilitic facts have been ol)- 

 sej-ved in the Mesabi range, but on a snialk'r scale and with a greater 

 distribution of detritus from cotemporary igneous rock. 



It appears, thererore. jliai bolli on ih(> W'rmilion and on the Mesabi 

 range the three forces that have been mentioned were jointly or suc- 

 cessively in action to produce the Minnesota iron ores, and also it is cer- 

 tain that tlie ores were produced immediately after the extrusion of lh(> 

 igneous rock. Th(\se agents were : 



1. Extrusion of basic igneous rock in immediate contact with oceanic 

 water. 



2. Sedimentary distribution of debris from the roek itself, and chem- 

 ical deposition of ferric oxide as well as iron carbonate among the other 

 sediments. 



o. Segregation of the iron, the silica, the alumina, and son«3times the 

 lime ccmtained in the basic rock, into separate masses, making both 

 residuary as well as ^constructive sliwts in the stratification. 



