340 THE CRYSTAL PALLS IRON-BE AEING DISTRICT. 



these rocks have all been folded, more or less strongly, at more than one 

 period ; and wherever they are exposed, they are seen to be mclined to the 

 horizon, often at hig-h angles, and to be traversed by intersecting sets of 

 joint-planes and cleavage-planes, some of which always cut the bedding, 

 and often have been the seat of the development of secondary minerals. 

 By the crossing of these various surfaces, the rocks are divided into small 

 unit masses, at the boundaries of which there is either an actual physical 

 parting or a break in the continuity of the magnetite. 



It is well known that when a bar magnet is broken and the severed 

 ends. are again joined, the two pieces do not unite to form one magnet, but 

 remain as two. If may be conceived, therefore, from the manner of distribu- 

 tion of the magnetite, and the secondary partings existing in these rocks, 

 that their magnetism is seated in an enormous number of small separate 

 magnets, at least one for each of the physically distinct unit volumes. 



It is a fact of observation, as will appear hereafter, that the upper 

 surfaces of these magnetic rocks invariably attract the north end of the 

 compass needles and, of course, repel the south end. From this it must be 

 inferred that the small magnets are generally similarly oriented, and have 

 their north ends, which would repel the north end of the compass needle, 

 pointing downward, and their south ends, which attract it, pointing upward. 

 As this is the arrangement that would result from induction from the earth's 

 magnetism, it can be (concluded further (as, of course, might be assumed) 

 that these rocks are magnetic from the earth's induction. 



It is also well known that when several bar magnets are joined in line 

 at opposite poles, the effect upon a compass needle within the range of influ- 

 ence is nearly the same as if the joined magnets were replaced by a single 

 magnet of the combined length. For each member of the pairs of inter- 

 mediate poles, one attracting and the other repelling, is about the same 

 distance away, and their effects so balance each other. The result, there- 

 fore, is to leave one pole unchanged in position and to remove the other to 

 the end of the last magnet added. If enough magnets are added, the final 

 result is to carry the moving pole so far away that it has no appreciable 

 influence upon the needle. This is a condition which, from the distribution 

 of the magnetite and the parting surfaces which run through the magnetic 

 rocks, must always be realized more or less completely. It is a necessary 

 consequence of such an arrangement of the small magnets that, in the case 



