AGE OF INTEUSIVES. 189 



duced subsequent to tlie folding of the Upper Huronian, or are of Kewee- 

 nawan or post-Keweenawan age. 



A closer approximation to the age of the intrusives is not possible, 

 unless we rely upon petrographical similarity. The dolerites of the Crystal 

 Falls district are similar to those forming the flows and dikes of the Kewee- 

 nawan on Keweenaw Point. They are also similar to the basic intrusives 

 of the Marquette district, with which this is practically a geological unit, 

 and likewise they agree petrographically with the dolerite dikes of the 

 Penokee-Gogebic district. In both districts the late intrusives have been 

 considered to be of Keweenawan age.^ Rominger,^ in Jiis report for 1881 

 and 1884, calls attention in a general statement to the possible connection 

 of the doleritic dikes peneti-ating the Michigan Huronian with the flows 

 and dikes of the "Copper-bearing formation" (Keweenawan). 



While correlation by means of petrographical similarity would not 

 hold for widely separated areas, it seems to be well worth considering for 

 areas which are so closely connected as are the iron districts of the Upper 

 Peninsula of Michigan. 



Judging from the evidence thus presented, the dolerites of the Crystal 

 Falls area are probably contemporaneous with the intrusions of the Penokee- 

 Gogebic and Marquette districts and with the volcanics of Keweenawan 

 time. 



EELATIOlSrS OF FOLDING AKD THE DISTRIBUTIOK OF THE 

 INTRUSIVES. 



In the preceding chapter, in the sections on folding of the Upper 

 Huronian, p. 158, it was shown that the main folds of the district follow an 

 approximately northwest-southeast coiu'se, and that upon these were super- 

 imposed minor folds approximately at right angles to these. The lines of 

 weakness parallel to the axes of the main folds Iiave been taken advantage • 

 of by certain of the intrusives, especially the dolerites. 



A glance at the general map (PI. Ill) shows that the dolerite dikes 

 which have been traced for considerable distances — that is, are more than 

 great knobs uncovered by erosion — have a northwest-southeast trend, in 

 agreement with the general direction of the major folding of the district. 



'Mon. U.S. Geol. Survey, Vol. XIX, p. 349; Vol. XXVIII, p. 218. 

 = Geol. of ilich.. Vol. V, cit., p. 6. 



