162 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



geneous volcanic and sedimentary Keweenawan series; (3) the upheaval 

 and truncation of the Keweenawan, in which movement of course the Upper 

 Huronian in the Keweenawan areas was likewise involved. Subsidence of 

 the land areas and the transgression of the Cambrian sea followed, with 

 dejxisition of the horizontal Lake Superior sandstone upon the inclined 

 Keweenawan and Upper Huronian rocks. The Upper Huronian of the 

 Crystal Falls district may have been involved in one or both of the foldings 

 which took place prior and subsequent to the Keweenawan; or, second, 

 since no Keweenawan deposits are known in the Crystal Falls district, it 

 may be that it suffered an early period of powerful orogeuic movement, 

 which raised the rocks above the sea, and was synchronous with the pre- 

 Keweenawan upheaval. A long period of erosion, accompanied perhaps 

 by other less important orogenic movements, may have followed contem- 

 poraneous with the activity of the Keweenawan volcanoes and the oscilla- 

 tory movements of the Keweenawan region. The latter I conceive to be 

 the more probable view. If this is correct, the intense folding of the Upper 

 Huronian sediments in the Crystal Falls distiict took place immediately 

 preceding the deposition of the Keweenawan series in other parts of the 

 Lake Superior region. 



RELATIONS TO OTHER SERIES. 



It has been seen that in the western part of the district the Hemlock 

 volcanics are the highest member of the Lower Huronian. At the end of 

 the volcanic activity there must have taken place a very general transgres- 

 sion of the sea, as is evidenced by the continuous belt of sedimentary rocks 

 which encircle the volcanics. The very marked change iii the character of 

 the rocks from subaerial volcanics to true sedimentaries partly marks the 

 division of the Upper Huronian and Lower Huronian series. The deter- 

 mining points in favor of this subdivision are found in the eastern part of the 

 district described by Smyth, and in the Marquette district still farther north- 

 east. In only one place in the western part of the Crystal Falls district, in 

 sec. 26, T. 44 N., R. 33 W., has a contact between the two series been obtained. 

 A drill hole here passed through a mottled slate just before entering the 

 Lower Huronian volcanics. A similar slate was obtained at Amasa over- 

 lying conglomeratic volcanic material, which outcrops at the surface, but 

 no direct contact has been found. With most careful examination I have 

 been unable to determine whether the conglomeratic rock is a true volcanic 



