366 DESCRIPTION OF THE 



columnar structure, the columns being from eight to fifteen inches in diameter, and 

 from four to six-sided, standing vertically, and with great regularity of outline. It 

 contains occasional nodules of laumonite and other minerals. The amygdaloid at 



this place does not differ from that described as occurring at other localities. The 

 cells are numerous and large, and many of them are filled with thalite, either incrust- 

 ing the sides of the cells, or in kidney-shaped nodules. Large nests of laumonite, 

 m conjunction with calcareous spar, are also common. These beds also contain 

 many thin veins of argillaceous iron ore, the sides of the veins being lined with 

 stellular crystals of heulandite. 



I did not examine the bed of the stream beyond the second ridge ; the latter is 

 three hundred and nineteen feet above the lake-level, and composed of a rock like 

 that which I have considered, at other localities, to be a volcanic grit. In 1848, 

 however, Colonel Whittlesey, who struck this river several miles above its mouth, 

 brought specimens of slaty hornblende, which he found in situ three miles from the 

 Lake. It may also be mentioned in connexion with this locality, that the amyg- 

 daloid in contact with the basalt, often assumes an imperfectly columnar structure. 



Below the mouth of Waginokaning River, the lake-shore is bounded by columnar 

 rock, which rises in rounded, dome-like exposures for more than half a mile. One 

 of these domes is represented in the following sketch. They rise from twenty to 



thirty feet above the water-level, and are surrounded by concentric layers of rock 

 on every side, like the coats of an onion. Over this rock is a bed of basaltic trap, 

 from three to fifteen feet in thickness, which seems to have been deposited since 

 the underlying rock began to disintegrate. 



The bedded traps in this neighbourhood appear to have been deposited at diffe- 

 rent periods of activity, with brief intervals of rest between them. At one point 

 on the lake-shore, about a mile above the mouth of Manidowish River, is an 



