674 W. 0. HOTCHKISS THE LAKE SUPERIOR GEOSYNCLINE 



bathylith rose from great depths within the earth slowly and through 

 long geologic periods; that it was rising during all Huronian time, and 

 played an important part in determining the character and thickness of 

 the Huronian deposits. This hypothesis is advanced to be criticized, to 

 see whether it is possible to relate all the major facts as suggested in this 

 paper or whether there are insuperable difficulties that may be brought 

 forward. 



SIZE AXD SHAPE OF THE BATHYLITH 



The size of the bathylith is indicated in part by the amount of material 

 extruded. The main part of the trap-rock s}Ticline — from near Minne- 

 apolis to the end of Keweenaw Point — is 300 miles long. It averages 

 about 80 miles in width. If the hypothesis here advanced is correct, the 

 average thickness of igneous rock would be well over 4 miles. This indi- 

 cates a total volume of the order of magnitude of 100,000 cubic miles — 

 enough to cover the Xew England States and the State of Xew York 

 with lava flows a mile in thickness. How much of the original magma 

 solidified within the chamber in addition to that extruded we have no 

 means of knowing. While this amount was probably large, it can be 

 neglected in our present consideration. Assuming that the bathylith was 

 elongated parallel to the axis of the present syncline and equal to it in 

 length, we have a cross-sectional area of at least 320 square miles 

 (4 X 80). While it is probable that the shape of the bath3dith in cross- 

 section varied greatly from time to time during its rise from the position 

 of origin, it seems most probable that its vertical diameter was much 

 greater than the horizontal. If the horizontal diameter was 8 miles, the 

 vertical averaged 40 miles. Any pair of diameters can be selected to 

 meet the reader's ideas of what is most probable mechanically. 



RISE OF THE BATHYLITH AXD ITS CONSEQUENCES 



The rise of the molten mass in the deeper portion of its travel must 

 have been approximately like that of a drop of oil rising in water — the 

 material above it flowing very slowly to the sides and that adjacent to 

 the lower part flowing in — a gravitative phenomenon, the molten mass 

 rising because it was lighter than the surrounding rocks. If the molten 

 rock were 2 per cent lighter than the surrounding solid rock (assuming 

 a specific gravity of 3.00), the differential pressure on the top of the 

 magma reservoir would be about 130 pounds per square inch for each 

 mile of vertical diameter of the fluid mass. There would be a minimum 

 vertical diameter of the fluid mass determined by the force with which 

 the overlying solid rocks resisted being pushed aside. If the vertical 



