92 GEOLOGY OF THE SECOND DISTRICT. 



side. A portion of a vein, on being cut tlirough by a dyke, is often moved to the right or left 

 the whole of its width, and frequently more. As these shifts occur at or near where it is 

 crossed by a dyke, it is generally supposed that they are the agents by which these derange- 

 ments are produced, though it is by no means clear that they are really the cause, inasmuch 

 as we must suppose a fissure existed prior to the injection of the dyke. It is true, that we 

 may conceive that the force which caused the molten rock to occupy and fill a rent, would 

 also act with some power upon the strata, and perhaps tend to increase the extent of the frac- 

 ture. Tlie shiftings and other derangements are not so frequent, neither are they so exten- 

 sive, in the iron region of the north, as in other mining districts. They rarely occasion any 

 inconvenience, or extra expense in the progress of working them. 



]t is an interesting fact, too, that though the matter composing a dyke never mingles itself 

 with the ore, yet it alters its character and quality. That portion of the ore which lies near 

 it, works differently, though no external difference appears to the eye, from that which is 

 taken from a distance. I believe this change is unfavorable ; that such ore is not reduced in 

 the forge so easily, neither does it make an iron of so good a quality as other parts of the 

 same vein. This fact favors the opinion sometimes expressed, that the kindness of working, 

 and the quality of metal, depend much upon the structure, and not altogether on the composi- 

 tion of the ore. 



4. Specular Oxide of Iron. 



Distinguishing characters, position, relations and disturbances occasioned by it and its 



associated minerals. 



The specular oxide of iron, as it occurs in the northern counties, is in two conditions : 1st, 

 that of a red powder, or red mass, whicii strongly soils whatever comes in contact with it ; and 

 2d, in brilliant, highly crystalline particles or crystals, with tlie lustre of polished steel. Not- 

 withstanding the external difference in the two conditions, they are the same chemically, the 

 iron in both being in a state of pero.xidation. Both varieties are found associated together 

 in the same mine : hence, it appears that the former is merely a disintegi-ated form of the 

 latter. It is readily distinguished from the magnetic oxide by its red streak, or the red color 

 of its powder, and by its want of magnetism. 



Form and Geological Position of the Specular Oxide of Iron. 



As in the magnetic oxide, so in the specular, there are two forms in which it is found : 1st, 

 that of masses ; 2d, that of veins. The former is the most common, and is the only one which 

 has furnished hitherto a suflScient quantity of ore for the supply of the different establisliments 

 for the manufacture of iron. In form and condition, the masses of specular oxide resemble the 

 magnetic ; there are no regular walls or regular boundaries, so far as they have been opened 

 and exposed to inspection. We find them, therefore, extending laterally rather than in depth ; 



