676 W. 0. HOTCHKISS THE LAKE SUPERIOR GEOSYNCLINE 



complete cessation of clastic deposition while the nearly pure chert and 

 iron oxide formation was being deposited. 



As the effect of this relief disappeared the eroded Archean rocks of 

 the dome wonld again rise and a renewal of clastic jieposition to the 

 north and south of the dome would follow. These deposits are the Vir- 

 ginia and T3der formations of the Mesabi and Gogebic districts. Pro- 

 gressive elevation of the dome and sinking of the trough during this 

 deposition explains the changing dip angles of the Huronian beds which 

 we find in the Gogebic district in going from the base to the top. At the 

 close of this period the basal beds of the Huronian dipped 20 to 25 

 degrees south and the top was level. 



The next major event was elevation of the surface, probably more 

 rapidly than at any previous time. The top of the Huronian rocks in 

 the Gogebic district was eroded and the eroded surface more or less com- 

 pletely covered with a new type of deposit — a thin land deposit of quartz 

 feldspar sand with pebbles of quartz porphyr}^, an oxidized yellow or red 

 formation, very different from the green, unoxidized sediments beneath. 

 This was quickly followed by a rapid outpouring of basic lavas. 



These last events are readily explainable by assuming that the magma 

 was nearing the surface and about ready to break through. The rapid 

 elevation and erosion at the close of Huronian deposition fits perfectly 

 with this assumption. The source of the quartz-feldspar sands and 

 porphyry pebbles can be found in the first extrusions of lava, which may 

 well have been acid. Being more viscous, these would not flow far to 

 the south and so might not be expected to be in evidence along the 

 present outcrop. Later outpouring of the more fluid basic material 

 would cover the Lower Keweenawan quartz-feldspar sands and porphyry 

 conglomerate. 



Lane and Gordon- have estimated that the acid eruptives evident in 

 the Black River section are about one-thirtieth as great in mass as the 

 basic eruptives. On the other hand, the acid sediments derived from 

 acid eruptives are about sixteen times as great in mass as the basic sedi- 

 ments and about four times as great as the acid eruptives. This must 

 indicate that close to the vents there were much greater volumes of acid 

 eruptives than are exposed at the greater distances where the series is 

 cut by the present surface. This is in accord with the common observa- 

 tion that acid lavas are less fluid and tend to pile up relatively near the 

 source. 



After the surface outpouring of lava began there were undoubtedly 



- Michigan Geol. Surv., Ann. Kept. 1906, p. 420. 



