62 Minnesota .lead any of Science 



1. Igneous Rock in the Mesabi Range. 



Near the close of the late Minnesota survey it was found, at 

 the east end of the Mesabi range, at Gunflint lake, that the Mesabi 

 rocks contained a considerable amount of volcanic elements. Some 

 of this volcanic element was in the form of ragged and rough pieces 

 of volcanic breccia mainly changed to flinty and jasperoid rock, 

 and some was yet glass — an old volcanic glass. As the study pro- 

 gressed, it was found that such volcanic glass was the main con- 

 stituent in the Mesabi at the west end of the range, really com- 

 posing the bulk of the so-called black slate. It was found that this 

 volcanic material had been rapidly accumulated, but that much of 

 it was in the form of sand, morii or less rounded by friction. It 

 was found that this volcanic sand had suffered alteration, at the 

 horizon where the ores exist in abundance, and by chemical changes 

 and transportion underground, had given rise to various new min- 

 erals. These new minerals were sometimes crowded together in the 

 strata, and sometimes were gathered in large amount in places by 

 themselves, and composed strata of considerable thickness. These 

 minerals are: — hematite, quartz, calcite, kaolin. 



The hematite, it is needless to say, is the soft ore of the Me- 

 sabi range. The quartz is the fine granular silica which has been 

 called (incorrectly) chalcedonic quartz. The calcite is that which, 

 in rare cases, constitutes thin and lenticular beds of limestone. The 

 kaolin is found also to constitute beds, several feet in thickness. 

 Now I have not mentioned several minor minerals such as actin 

 olite, sphene, mica, which are in microscopic amounts, but they 

 ought to be mentioned because they are characteristically produced 

 by the alteration of basic igneous rock. 



I have omitted, also, to mention another important product of 

 this alteration, viz: a green, rather soft, substance which has been 

 named greensand and greenalite. 



This igneous character of the original rocks of the Mesabi range 

 has recently been discussed anew in the "Proceedings of the Lake 

 Superior Mining Institute." Some of the characteristic outward 

 aspects of this rock will be shown. 



(20) The basaltic jointage seen in the rock cut by the rail- 

 road in the approach to the Oliver mine. Nothing but the cooling 

 of heated rocks is known to produce such a jointage. It can be seen 

 in any place where massive igneous rock has been allowed to cool. 

 It sometimes occurs also in non-igneous rocks that have been heat- 

 ed by contact on igneous rocks, as in the conglomerate of Ogishke 

 Muncie where affected by intrusive granite. This is an infallible 

 sign of great heat and slow cooling. 



(21) As the igneous rocks of the Mesabi range were largely of 

 the nature of surface lava, so several of the structures of trap rock 

 have been observed. This view gives an illustration of the struc- 

 ture known as "ropy structure." When a mass of liquid lava has 

 become cooled so as to be covered by a thin scum or skin, if the 

 molten part continues to move this skin of semi-cooled lava crum- 

 ples up on the surface in the same manner as the crumpling of 

 cream at the edge of a pan as the milk flows out from under it. 

 Such a crumpled mass of trap may be several inches in thickness. 

 The view shows a small fragment. 



