124 C. B. Van Ilise — Iron Ores of Michigan. 



space between may be occupied by ore. When a dike is alone 

 and has a flat dip the ore is always on the upper side, that is in 

 such cases the dike rock is always the foot-wall of the deposit. 

 (See fig. 1.) 



A. dike carrying ore may unite with a large mass of soap- 

 stone varying to»diorite, which also carries ore, when a trough 

 will be formed and the deposit is here apt to become large. In 

 this case we have a union of the second and third classes of 

 deposits. At the locality shown in fig. 3 the soap-rock grades 

 into diorite and dips south about 45°, constituting the main 

 foot-wall. Standing vertical and striking 20° or 30° away 

 from the foot-wall is a dike of paint-rock or soap-rock cutting 

 across the formation. Resting upon both are ore-deposits, and 

 by their union at the apex of a trough formed by the junction 

 of the two the ore-body is of large size. 



(4-.) Deposits interoedded in the jasper or chert. — No de- 

 posits of this class of large size are known, for unless below is 

 a soap-rock (when it would fall into a previous class) no imper- 

 vious stratum is present upon which the ore can collect, unless 

 (and this does not often occur) a layer of the ore-bearing 

 formation itself locally loses porosity. 



The ore-deposits secondary concentrations. — The ore-bodies 

 of the second, third and fourth classes, that is all that lie 

 wholly within the ore-bearing formation, are usually soft, and 

 they may occur at almost any horizon within the iron-bearing 

 formation. Being for the most part at some distance from the 

 contact between the quartzite-conglomerate and ore-formation 

 they are commonly within the ferruginous chert rather than 

 in the banded ore and jasper. 



The foregoing descriptions show how intimate is the connec- 

 tion between the ore-bodies and the paint or soap rocks. Mr. 

 Thompson says this connection has become so evident to the 

 miners, and they have such confidence that this material is a 

 favorable indication, that it is called by them " the mother of 

 the ore." 



The ore- deposits in longitudinal sections are not horizontal, 

 but generally have a pitch, which in the Ishpeming-Negaunee 

 area is usually to the west and often amounts to as much as 20° 

 or 30°. Consequently the ore-bodies represented in horizontal 

 cross-section are found at different depths in following them 

 longitudinally. This principle of pitch is equally applicable 

 whether the underlying formation is jasper or soap-rock, 

 whether the ore is in sheets, lenses or troughs. 



While the ore-deposits of the Lower Marquette series have 

 a t greater variety of form and relations than do those of the 

 Penokee district, it is evident that the conditions governing 

 their formation are much the same. In both districts the 



