TERRESTRIAL MAGNETISM. 



13 



Dip. 





Epoch of 



Duration 





Calculated 



Square of the 



Horizon- 



Total in- 



Total inten- 





commencing 



of 500 



IB 



duration 



preceding. 



tal inten- 



tensity : 



sitv : that at 



!^ a 



vibration. 



vibra- 



at 60°. 





sity. 



hor. being 



Cincinnati 



hrs. min. sec. 



tions. 











1000. 



= 1000. 



168. Brandywine River, near Wilmington. May 31, 1846. 



67° 30' 00" 



11 2 00.0 A 



1053.0 



74.0 



1052.00 



11067040000 1113..54 



2909.8 



976.11 



169. Sawyer, Del.— Lat. 39° 42' 53" N. Long. 75° 42' 39" W. June 3, 1846. 



71° 57' 26" 

 Var. 2 47 48 W. 



11 38 20.0 A 



12 19 02.0 P 

 12 49 02.4 



1160.0 79.0,11.58.57 

 141(1.0 s:^,.0 1407.73 

 1404.0 84.5 il401.00 



13422844440 918.11 





198170:i7;V20' 917.70 





19630251GG4 920.60 





Mean 918.80 



2966.50 



995.11 



170. Church Landing, N. J.— Lat. 39° 40' 56" N. Long. 75° 33' 23" W. June 6, 1846. 



71° 23' 58" 

 Var. 5 45 47 W. 



11 05 01.2 A 

 11 37 17.0 

 1 03 01.2 P 



1142.0 



1386.8 

 1382.0 



83.0 

 81.5 



82.5 



1140.30 

 1384.71 

 1379.26 



13002840900 



947.76 





19174217841 



948.46 





19023581476 



949.96 





Mean 



948.73 



2969.66 



996.06 



REMARKS. 

 So strong is this polarity in a gnecimen procured near the Wilmington station, that I was enabled, by a 

 dexterous application of it towards the suspended dipping-needle, to bring that needle into an oscillation 

 through more than half a circle. This specimen was found to contain small grains of black oxide of iron 

 similar to those which constitute "black magnetic sand." 



From Wilmington, I addressed a note to the Amer. Phil. Society, on the subject of the effects of 

 magnetic iron sand in the trappean rocks. Indeed, the trappean rocks, so far as I have observed, seem 

 to be the source and fountain of this sand. I found it abundant at the foot of the hills about Wilmington, 

 where it had evidently washed down from the disintegrated trappean rocks forming the elevations. It 

 abounds in the trap-rocks of Lake Superior, and can be gathered on all of the great lakes below, even upon 

 the south shore of Lake Erie. The diluvial drift along the Ohio River contains it, and, after a shower, it 

 can be collected by a magnet from the rain trenches formed in the fine sand. Dr. Jackson found that 

 when the trap-rocks were smelted to procure copper which they contained, a button of pure iron was not 

 unfrequently found as a result. The black magnetic sand, distributed from Lake Superior to Cincinnati, is 

 accompanied by a garnet sand of a deep brown color and with translucent particles. 



168. Brandywine River. — Locality: in a deep recess, almost a cavern, in the trap-rocks near the 

 water's edge, about one-third of a mile above Riddle's Mill. This locality is not more than a half mile 

 distant from " Smith's Quarry," and not over two miles from the Wilmington signal. Geology: the same 

 as at Smith's Quarry. A strong local influence is here evinced by the extraordinary difference of the dip. 

 Still, the law that I have heretofore alluded to is observed, viz., an increased dip at trappean pinnacles 

 (Smith's Quarry), and a diminished dip at the base (Brandywine River). This is the most extreme case 

 of the kind, the difference of dip being 6° 41' 41" in a distance of half a mile. 



169. Sawyer, Del. — Geology : at the edge of the tertiary. No rocks or bowlders apparent. 



170. Church Landing, N. J. — Geology: tertiary sands of New Jersey. No rocks; nothing especially 

 magnetic apparent; still, there are local influences, for the declination is extraordinary, and, by partial trials 

 at three localities, distant each about half of a mile, it was found to vary about 1°. From this, I infer that 

 the magnetic rocks of Wilmington extend a moderate depth under the tertiary deposits at this place. 



