Magnetism of the Earth. 145 
transparency, and. are sometimes levelled upon their acute lateral 
edges. 
4. Tabular Spar and Idocrase at Moriah, (N. Y.) 
These minerals were found by Dr. Lewis Heermann and myself, 
during a mineralogical excursion upon the western shores of Lake 
Champlain. ‘The Tabular Spar we found in loose blocks, in a wood 
by the road side, about one mile west of Major Dallarby’s. It oc- 
curs in foliated prismatic masses of considerable dimensions, highly 
translucent, having a greyish white color with a tinge of olive. In- 
termingled with it is a brown mica, and more sparingly, scales of 
Plumbago. We did not find it zn place ; but, from the nature of 
the rocks in the vicinity, we were disposed to consider it as coming 
from the primitive limestone. The Idocrase is found near Maj. Dal- 
larby’s mills, and occurs among the rocks thrown out in forming a 
mill-sluice. It is of a dark reddish brown color, and in masses of 
considerable size, one of which we obtained, presenting several crys- 
talline facets. It is associated with iron pyrites, calcareous spar, and 
the delicate hair brown hornblende, which elsewhere in the United 
States, so frequently accompanies the spinelle. 
Yale College, Sept. 8th, 1829. 
Arr. XVI.—Observations on the Magnetism of the Earth, especial- 
ly of the Arctic Regions ; in a letter from Capt. Edward Sabine, 
to Professor Renwick. 
Communicated for this Journal. 
TO PROF. RENWICK. 
My dear Sir,—I received a few days ago, a letter from Professor 
Hansteen, of Christiania, dated from Irkutsk, in Siberia, in April last. 
. Hansteen is travelling, as you know, at the expense of his iXing, 
and with the permission of the Emperor of Russia, for the purpose_ 
of observing the Magnetic Dip, Variation, and Intensity, over the 
whole of the north of Europe, and of Asia; and of comparing the 
actual phenomena with the system of terrestrial magnetism, propound- 
ed by himself, in his celebrated treatise, entitled “ Magnetismus der 
Erde.” aes 
Vor. XVII.—No. 1. 19 
