88 GEOLOGY OF THE SECOND DISTRICT. 



one class of principles ; but still, externally, the two oxides are quite dissimilar, and for a 

 detailed account, seem to require a separate consideration. 



Geological Position. 



The magnetic oxide is found in two geological forms : 1st, in that of masses ; 2d, in that 

 of veins. In the northern counties, we have numerous examples of both. I would remark 

 here, that I have used the word mass, not in its usual meaning, but as expressive of a par- 

 ticular form, or mode in which this ore is found. It is a mineral inclosed in rock, in an 

 irregular manner, so far as its connection is revealed : there are no regular boundaries ; it 

 projects into the surrounding rock, and becomes more or less incorporated with the adjacent 

 portions. To see more clearly what is meant by a mass, it may be contrasted with a vein, 

 which is a space of a certain width and direction, filled with mineral matter, but with boun- 

 daries clearly defined by what are termed walls, which are formed of the fractured rock itself, 

 and upon the whole are considered parallel, though at many different places the parallelism 

 does not exist. The word mass may be confined to the form of occurrence, as defined above ; 

 reserving the word bed for those forms when they occur in the earth, or in the midst of soft 

 materials, as beds of hematite, manganese, etc. 



In those forms which I have called masses, there is probably much to be learned by future 

 exploration. As we are unable to see their full extent, we are unable to determine their pre- 

 cise nature. We, in fact, do not know but that they are extended rocks of ore, beneath all 

 the others of wliich we have any knowledge. 



Of the two forms in which the magnetic oxide occurs, I shall mention first, that which I 

 have called a mass, which I have defined a quantity of ore inclosed irregularly in the midst of 

 rocks, whose extent laterally, or in the direction usually called width, is unequal, and whose 

 lines of demarcation between the ore and rock are indistinct and ill defined. Of this form are 

 the ores of Adirondack, in the west part of Essex county. I liave, at different times, enter- 

 tained different views in regard to the form in which these ores occur ; at one time calhng 

 them beds, and at another, veins, neither view being satisfactory. I have at last called them 

 neither one nor the other ; adopting the above as more expressive of their appearance than 

 either of the others. Should I attempt to describe these masses, I could convey an idea of 

 them in no better language than to speak of them as ledges, cliffs, or rocks of iron ore ; 

 exhibiting the same structure, natural joints, or divisional planes, as other rocks. Such is 

 certainly the structure in the midst of the mass ; but when the ore is situated near the rock, 

 it gradually takes in a greater proportion of earthy matter, or portions of tiie rock, and per- 

 haps becomes incorporated therewith ; but at- other times, it sends out branches into the 

 adjacent rock, as in the annexed sketch, which though they often appear to be cut off from the 

 main mass by the interposition of rock, yet can in most instances be traced to it by oblique 

 prolongations, as seen in the cut, (fig. 21.) In other instances, the mass passes clearly 



