94 



GEOLOGY OF THE SECOND DISTRICT. 



taken in connection with those which have been aheady given of the nature of serpentine, 

 carries with this view a high degree of probabihty. 



There is one point of great tlieoretical and practical importance, which it is proper to slate 

 in this place ; it is, the great difference in the quantity of ore contained in masses at different 

 places. Thus, at one place it may extend only a few yards, or consist of only four or five 

 hundred pounds, and in some instances even less ; while in others it forms wide extensive 

 beds, whose limits are not yet ascertained. In all those instances, I have observed a connec- 

 tion with a supposed igneous rock, as serpentine, or primitive limestone. 



Another interesting fact is disclosed at some of the leaner beds of this ore which have been 

 opened at Theresa. The ore is arranged in indistinct layers in the Potsdam sandstone ; these 

 layers appearing to have been originally this sandstone only, but subsequently filled or satu- 

 rated with the oxide of iron, some of which, if found in any other place, as in boulders, would 

 have passed for quartz highly charged with iron. But at the locality referred to, there are 

 rich masses of ore interspersed with the lean ; and withal, serpentine appears too, pushed 

 into the mass, and occupying an important place, as in the Parish bed. The third diagram 

 of this series, (fig. 26,) illustrate the observations I have just made. 



a, Serpentine ; b, b, Specular oxide. 



In fig. 26, there is the same disturbance as at the Parish and Kearney beds ; the fracture 

 of the sandstone, its elevation, and anticlinal dip. This diagram illustrates the position and 

 relations of the masses at the Shirtleff bed, four miles east of Theresa falls. 



I can offer but little besides conjecture of the extent of the masses of the specular oxide, as 

 they are revealed by uplifts along or near the borders of the primary and transition formations. 

 The sandstone is of course thin and irregular in its extent ; and the disturbances which have 

 taken place, ■ are all favorable for bringing to light masses of ore which may exist in connec- 

 tion with it. As we recede from this border towards the deeper portions of the transition 

 rocks, the fractures are less in extent, and rarely, if ever, sufficiently great to bring to light 

 beds of mineral matter which may be interposed between the two classes of rocks. 



As an additional illustration of the position of this ore, I have added fig. 27, where the mass 



