GEOLOGY. 255 



some enormous mass of iron, or (if the expression be allow- 

 ed) some ' subterraneous iron-mountain,' which may resem- 

 ble, except in position, that of Missouri. The locality indi- 

 cated is on the Wapsipinecon ; and the axis of this node, as 

 Dr Locke's chart show^s, is near the line dividing townships 

 eighty-two and eighty-three, and about six miles west of the 

 hfth principal meridian. 



" The utility of magnetical observations on the dip and 

 intensity of the needle, as an indication of the presence of 

 iron, and perhaps, also, of great masses of the brown oxide, 

 is indisputable ; and I consider myself fortunate in having 

 been able to add to the other materials whereby to decide 

 the value of the various locations of mineral lands in this 

 district, the dehcate and varied experiments of Dr. Locke. 



" The variation-chart appended to that gentleman's report 

 shows a striking difference in the variation of the needle 

 within a very short distance ; and the greatest variation cor- 

 responds, in a remarkable manner, with the best locations of 

 iron ore of which actual discoveries were made. If from 

 this we may conclude that the variation is increased by the 

 presence of large masses of ore, the above chart would use- 

 fully guide a further examination after the localities of iron 

 ore in the district. It must, however, be remembered, that 

 it is the protoxide which chiefly acts upon the needle, and 

 that the same phenomenon may possibly be caused by com- 

 paratively small veins of that variety, as by a large mass of 

 the brown oxide. 



" The richness of the iron-veins in this district cannot be 

 correctly known until mines shall actually be opened ; which 

 has not yet been done in any part of it. But more encourag- 

 ing or more numerous surface-indications of an abundant 

 supply of this useful metal can hardly offer themselves to 

 the notice of the geologist. In a country more thickly set- 



