312 OBSERVATIONS ON THE MAGNETIC DIP, 
tible of change by heat, I was unable to determine. At Copper Harbour the strata dip 
very regularly to the northward, at an angle of about 30°, and consequently they plunge, 
on one side, under the lake itself; and rising on the other side above the waters form a 
rocky slope well calculated to receive, lift up, and gradually destroy the mighty waves 
which the strong north-west wind often throws upon the coast. These two characters, 
the alternation of soft and hard layers, and their inclination in such manner as to bring 
those layers alternately to the surface, afford the proper key to the topography of the 
coast. It is evident that the lake must necessarily act more rapidly on the softer than 
on the harder strata, and that the waves will, by breaking over, or by a narrow breach 
through, the barrier formed by a harder stratum, break down and remove a soft 
stratum interiorly, as it were, and thus form elongated bays with but a narrow inlet. As 
the action proceeds, the hard stratum itself giving way, at numerous points, will be cut 
up into numerous islands, situated in a chain, still protecting a bay or harbour within. 
These islands, too, when seen endwise, present, with great uniformity, the appearance 
of saw-teeth, rising, as they do, gradually on the slope next to the lake, and dropping 
abruptly on the side next to the shore. These characters areewell exhibited at Copper 
Harbour and at Agate Harbour, a few miles to the westward of it. Copper Harbour 
itself seems to have been formed by the removal of a softer stratum of metamorphic 
sand rock, while Porter’s Island is a part of the barrier, formed by the outcropping of a 
harder layer. 
DESCRIPTION OF THE PLATES. 
Plate No. 43, is a chart of the United States, on a conical surface, passing, unrolled, 
through the parallels of twenty-five and fifty degrees of north latitude. I have since had 
occasion to prefer a chart on a tangent to that on a chord, the difference would have been 
that the meridians would have been somewhat more distant than they actually are. The 
object of this chart is to exhibit to the eye the approximate configuration, especially of the 
isodynamic lines; farther than this I attach no especial importance to them. It appears 
that they form ellipses more or less concentric; a fact anticipated, and clearly pointed 
out by Colonel Sabine, in 1838, from very distant surrounding observations made on 
the Atlantic, Pacific, and at Baffin’s Bay. My observations have fixed, more precisely, 
those lines, have determined the value of the intensity at. particular localities, and have 
shown that the centre, or axis of greatest intensity, is at, or near to, Lake Superior. 1+ 
will be seen by the results of my researches, engraved in numbers on the chart, how far 
I have actual authority for the chart lines. So far as they extend beyond the limits of the 
United States, and into the British possessions, they are projected with nothing more than 
a general consistency with distant observations. The researches conducted under the 
government of Great Britain will presently settle that part of the chart, in a most perfect 
manner. ‘The unit assumed for the expression of the total intensity is that of Cincinnati, 
which has been called 1000. The line passing through New York was found to have the 
intensity expressed by 994, and from this proceeding inwardly, towards Lake Su perior, 
lines have been drawn at 1004, 1014, 1024, &c., having a common difference of 10. Upon 
every line, [ have also marked its value according to Baron Humboldt’s unit, adopted 
by Colonel Sabine, in his Report of 1838, which I have designated by the name 
Sabine, or the initial S. 
