18 OBSEKVATIONS ON I. 



of the Secretary of the Smithsonian Institution, I was appointed Dr. Jackson's 

 assistant, with the understanding that I was to perform both magnetical and geolo- 

 gical duty. It was also the intention of the Hon. Secretary, Mr. Walker, as I am 

 informed, that the magnetical observation should be communicated to the Smith- 

 sonian Institution. The rules of subordination, however, did not permit me to do 

 otherwise than to report them to my superior; and, as Mr. Walker's term of office 

 had in the mean time expired, the observations took their course, and were incor- 

 porated with the documents of Congress, and published without my supervision. 

 By an oversight of my own, which I should have corrected instantly, had I seen 

 the proof-sheets, the headings of the tables were omitted, and they were thus ren- 

 dered unintelligible. Tl ere is, therefore, not only a propriety, but a necessity, for 

 appending them to this communication, in order to their repubhcation. 



I have incidentally mentioned the connection of terrestrial magnetism with geology. 

 This is intimate and j)ractically important, and I have been particular in all my 

 researches to note the geology of the station. I have elsewhere noticed that primi- 

 tive or igneous rocks, and especially trappean rocks, are accompanied by extraordi- 

 nary and evident magnetical effects. Nowhere in my researches are these effects 

 more conspicuous than in the trappean developments of the region of Lake Superior. 

 In the aqueous formations of the great valley of the Mississippi and its tributaries, 

 where the rocks are horizontally stratified, the elements of terrestrial magnetism are 

 so regularly graduated that, after making a few observations, and learning the rate 

 of increase or decrease of any magnetic quantity corresponding to known distances 

 and directions, one is enabled to calculate the one from the other — the magnetism 

 from the distance, or the distance from the magnetism — ^with an approximate accu- 

 racy, within moderate distances ; yet not, as some have supposed, with such preci- 

 sion as is reliable for the geodetic accuracy of latitude and longitude. But in the 

 trappean regions of Lake Superior, the magnetic quantities are changed very 

 abruptly in very short distances, and that, too, according to no known law. Yet, 

 even here, when one passes from the upturned conglomerate alternating with, or 

 traversed by, dikes of trap, and, perhaps, containing metalHferous veins of copper 

 and silver, to the horizontally stratified sandstone — as at the Apostles' Islands or the 

 Pictured Rocks — the magnetic elements become again consistent, and increase or 

 diminish according to magnetic latitude and longitude. So constant and evident 

 were these effects, that Dr. Houghton and Judge Burt, in their land surveys of 

 those regions, relied upon a single magnetic element, the declination (variation), to 

 indicate the general geology of rocks concealed under the soil of the tangled forests. 

 Whenever the needle became so abruptly deflected by local attractions as to render 

 the running of the lines by magnetic direction difficult, then they ventured to note 

 "trap-rocks;" and the researches of subsequent geologists have not found any error 

 in their decisions, thus founded on a single element of terrestrial magnetism. Some 

 extraordinary magnetic manifestations, in connection with metalliferous veins, will 

 be pointed out in the course of this paper. 



Although the local variations of magnetism seem, in general, to follow no known 

 law, yet the reader will see, by referring to the series for 1844, published in the 

 "Transactions of the Am. Phil. Society" for 1846, that trappean hills and pinnacles 

 have a special magnetism, the magnetic axis coinciding with the axis of form, being 



