0 N THE NORTHWEST S H 0 R E O P LAKE S TI P E R I 0 R. 403 



their course in the valley beyond. The next ridge is traversed in like manner. 

 The following sketch of one of these dikes, as it starts from the lake-shore, will 

 convey a much better idea of their general appearance than a written description. 



EXPOS Ell COLUMNAR BASALTIC DIKE, LAKE SUPERIOR. 



They vary in width from eight to sixteen feet, and are generally perpendicular, 

 though some of them hade slightly. Thin veins of calcite and seams of steatitic 

 material are occasionally interposed between the sides of the dike and the slate w r alls ; 

 and sometimes there are evidences of a slip or fault in the sedimentary beds. In 

 many of the veins the slaty wall-rock is broken into small fragments, and dissemi- 

 nated through the calcite. When these veins branch off into slates, as is sometimes 

 the case, they are broken into strings, and soon lose themselves. If they are con- 

 tinued sufficiently far, however, to reach another dike or bed of trap, they again 

 become concentrated. 



TRAP OVER SLATE, LAKE SUPERIOR. 



Sometimes the slates rise in mural escarpments to the height of two hundred 

 feet, and over these a bed of trap is imposed. Where the dikes have been broken 



