THE LOWER HURONIAN. 283 



adjacent. Moreover, in places, as we get farther from the eruptives, the 

 gradation from the conglomerate to finer-grained sediments can be dis- 

 tinctly traced, showing plainly that the conglomerates were derived from 

 the eruptives, and that consequently they are of later age. The conglom- 

 erates and eruptives especially have been very closely infolded and they 

 now show the most intricate surface relations. The large-scale detailed 

 maps of the east end of Ely Island (fig. 18), and of the point south of Mud 

 Creek Bay (Sheet XXV of Atlas), will give some idea of the intricacy 

 of their surface relations and will indicate the difficulties of successfully 

 determining the geologic structure. This intricacy of relationship between 

 the sediments and eruptives is in some places most puzzling. A sketch, 

 fig. 19 (p. 290), made in the field, illustrates the relations between these 

 two' rocks which were seen on an exposure on the north side of Ely Island, 

 near the east end, and which will be described in some detail further on. 



The character of the conglomerate is usually well marked, especially 

 when jasper pebbles are abundant in it. Under such circumstances the 

 pebbles of the acid igneous rocks in the cong'lomerate can also be identi- 

 fied with the adjacent masses of granite and porjDhyry. But where both 

 the igneous rocks and the conglomerate derived from them have been 

 very much mashed, and especially where there is a comparatively fine- 

 gi'ained sediment — in other words, a grit — it is very difficult to discrim- 

 inate between them, for both the igneous rocks and the grits derived from 

 them have been sheared into white to gray fissile sericitic schists which 

 have practically the same appearance. It is not improbable that some 

 mistakes have been made on the detailed maps in this discrimination, but 

 extreme care has been exercised, so that the mistakes are unquestionably 

 few, and while they may affect the determination of the areal distribution 

 of these rocks they are not of such character as to affect the interpretation 

 of their general relations. 



Relations to the Giants Range granite. — South of Tower, in the -s-icinity 

 of milepost 92, on the Duluth and Iron Range Railroad,' the Knife Lake 

 slates are intruded and metamorphosed by a number of granite dikes which 

 have been correlated with the Giants Range granite. The fact that this 

 granite is younger than the Lower Huronian sediments is thus clearly shown. 



Relations to haste dikes. — The Lower Huronian sediments, both the 

 Ogislike conglomerate and the Knife Lake slates, are cut by occasional dikes 



