THE EEUPTIVES. 347 



ties which are ordinarily taken as indicating- that the rocks possessing 

 them have soHdified at depth. 



The presence of very numerous dike-rocks in the iron-bearing beh has 

 become apparent tlu'ough exploring and mining operations. These dikes 

 are much altered and soft, and therefore they do not outcrop, so that their 

 existence would never have been otherwise suspected. In almost every 

 mine in the whole district, and in many test pits, dike-rocks have been en- 

 countered. These vary g'reatly in thickness, running from but a few 

 inches to 90 feet. The positions of these dike-rocks are g-iveu in detail in 

 the discussion of the origin of the ores ; and' here it is only necessary to 

 say that in general they cut across the u'on fonnation nearly at right angles, 

 and therefore were probably formed before the uplifting- of the Penokee- 

 Grogebic series. In quite a number of cases they have been traced into 

 the underlying- quartzite. Frequently tlie places of passage of the dike- 

 rocks from the iron formation into the quartzite show faulting- to a greater 

 or less degree. This fiiulting along the line of dikes is illustrated by PI. 

 XXXI, Fig. 6. 



In the Upper slate member the greenstones outcrop with sufficient fre- 

 quency to show that they are quite plentiful, but it has not been possible 

 to detei'mine whether they are interleaved or are in the nature of dikes. 

 It is not at all unlikely that both dike-rocks and sheets occur. Indeed it 

 Avould almost certainly be the case that some material from the nu- 

 merous dikes which pass through the iron ])elt would succeed in intrudino- 

 itself between the layers. A large exposure on the Wisconsin Central 

 Railroad, in the NE. \ of the NE. \ of Sec. 8, T. 44 N., R. 2 W., Wiscon- 

 sin, gives indications that a certain amount of interlea\'ing has taken 

 place, the greenstones and slates being mingled, however, in a confu.sed 

 manner. The outcroj) occurs just north of the railroad, and in passino- 

 eastward mixed greenstone and slate appear. There are several belts of 

 almost pure slate i-ontained in the greenstone, Init after a time the slate is 

 continuous. If the eruptive is a great dike, it may be that slate fragments 

 have been caught by it ; but it apjjears more probable that it has squeezed 

 itself between the layers of the slate adjacent to the contact. The line 

 between the Upper slate member and the overlying Keweenaw series is 



