HYPOTHESES EXPLAINING THE FACTS 677 



alternating periods of slow and rapid extrusion. Some portions of the 

 main volcanic range would be quiescent while others were active. In 

 any given part of the range there would be periods of activity and periods 

 of quiescence. D|^lring the quiescent periods the magma beneath would 

 have time for partial . differentiation. At the surface erosive agents 

 would find time to spread acid conglomerate and sandstone beds over 

 the flanks of the range, the material for which they found in the acid 

 flows in the higher areas. When extrusion began afresh, the character 

 of the flows would differ, more or less, from the preceding flows to corre- 

 spond with the degree of differentiation that had taken place. This 

 variation in character is strikingly brought out in Mr. Aldrich's magnetic 

 work in northern Wisconsin this past summer. 



During the time of the extrusion there was a slow progressive sinking 

 of the range which nearly kept pace with the thickness of the extruded 

 flows. This sinking was probably one of the chief causes of the con- 

 tinuance of extrusion — the load sank into the magma reservoir and 

 squeezed the magma out. This sinking affected the area for many miles 

 on either side of the vents and included the Huronian rocks and the 

 Archean rocks to the north and south. The ancient Archean baselevel 

 in the Gogebic district was thus tilted back to a horizontal position, then 

 northward, so its present dip is about 60 degrees to the north. 



After a moderate thickness of the lower basic flows had been extruded, 

 numerous dikes found their way along tension cracks into the strike 

 joints of the Huronian. At a later date these dikes were sheared by a 

 great fault, an eastward and upward thrust of the upper part of the 

 series parallel to the beds. Lane^ has suggested that this may be corre- 

 lated with the intrusion of the gabbro laccolith in Wisconsin. This in- 

 trusion may also be contemporaneous with the development of the great 

 Keweenaw thrust-fault, and the two upward thrusts of the north part of 

 the series on the Gogebic may be a part of the same general readjustment. 

 This could be accounted for mechanically as a gradual differential sink- 

 ing of the outer rocks into the magma reservoir to force more lava out 

 of the vents. These fault adjustments probably began when the north- 

 ward dip of the lower flows was less steep than at present. 



At the time of the final extrusion there was most probably a high 

 central range largely composed of acid flows. From this range was de- 

 rived a goodly part of the Upper Keweenawan conglomerates, sandstones, 

 and shales which have been estimated by Thwaites* to have a maximum 

 thickness of nearly 25,000 feet. 



^ Loc. cit., p. 470. 



* Wisconsin Geol. and Nat. Hist. Surv., Bull. xxv. 



