. The Crystal Falls iron-bearing district of Michigan. Iron ores; Geology; Geology, Stratigraphic; Geology, Stratigraphic. MAGNETIC OBSERVATIONS. 371 Thereftiro, in yoneral, tliL- l)uried north poles of the magnetic formation are not brought nearer the surface; and this, together with the fact that the material continues on in the line of the axis, jiroduces characteristic phe- nomena in the magnetic sections. These phenomena, "the details of which can be easily followed out for any given direction of pitch, and need not here be described, show in gen- eral two lines of attraction luergin
. The Crystal Falls iron-bearing district of Michigan. Iron ores; Geology; Geology, Stratigraphic; Geology, Stratigraphic. MAGNETIC OBSERVATIONS. 371 Thereftiro, in yoneral, tliL- l)uried north poles of the magnetic formation are not brought nearer the surface; and this, together with the fact that the material continues on in the line of the axis, jiroduces characteristic phe- nomena in the magnetic sections. These phenomena, "the details of which can be easily followed out for any given direction of pitch, and need not here be described, show in gen- eral two lines of attraction luerging into one, which continues in the same direction as a strong line, showing, as it is followed, the peculiarities due to an increasing deptli of burial. The points of maximum deflection of the liorizontal needle continue to separate from each other on suc- cessive sections. The dip curve shows a definite maximum closely corresponding, except for due east-and-west strikes, to the point of no horizontal deflection. Where the axis of tlie fold is so oriented that these points can be , they indicate the nature of the fold. If the strike is east and west, in which case they become indeterminate, the continuity of the line and its A'ery gradual decrease in power may give an excellent basis for inference as to the nature of the r^7n\ A Fig. 23. CROSS SECTIONS -Pliiii aud cruss sectioBs of a pitching anticline. 3. FORMATIONS SPLIT BY INTRUSIVES. When a single formation has been split into two by the intrusion of a nonmagnetic igneous rock, there are in the area in which the igneous rock occm-s two pai-allel magnetic formations, which give rise on cross traverses to phenomena the precise features of which depend upon the strike and dip of the formation and upon the relation which the width of the intruded mass bears to the depth of burial. To describe these would involve a mere repetition of what has been said before. Such intruded masses always have a definite limit
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