ON THE NORTHWEST SHORE OF LAKE SUPERIOR. 391 



feet thick. At one point there appears to have been a depression in the sedimentary 

 rock at the time of the overflow, and in this depression a gutter has been formed, 



a. Shaly amygdaloid, b, b. Shale, with clay seams, c, c. Shale, with nests of minerals, d. Quartzose porphyry. 



filled with a breccia composed of small fragments of sand-rock cemented by the trap. 

 The gutter extends into the Lake, and can be traced for some distance from the 

 shore. The following sketch, by Major Owen, exhibits the section at this place. 



SHEET OF TRAP OVERFLOW, LAKE SUPERIOR. 



The sides of the gutter are from six inches to three feet in height. The sand- 

 stone (No. 88) immediately under the breccia is very much altered, and looks like 

 sandstone burnt in a furnace. That which underlies the trap-bed is less altered 

 (No. 89), but assumes a somewhat trappous appearance at the contact. A portion 

 of the bed near the gutter has the appearance of jasper. (No. 87.) The direction 

 of the gutter is from north to south. The first high ridge is about a mile and a 

 half north of the lake-shore. 



A short distance further on, is an exposure of siliceous shale (No. 90), overlaid 

 by altered sandstone (No. 89) ; both rocks dipping southwest at an angle of Vl°. 

 Just above the long point below the mouth of Manitobimitagico River, and near the 

 mouth of a small stream which comes in there, are some low exposures of No. G37. 

 At the point itself, the trap overlies a breccia, from eight to ten feet in thickness, 

 and filled with zeolites. These rocks continue to form the shore as far as the mouth 

 of Manitobimitagico River, a short distance above which a trap ridge strikes the 



