■ MICHIGAN ACADEMY OF SCIENCE. 101 



<'onditions. This accounts for the accumiihition of ferric oxide, so freciuently 

 noted, in the residuum produced l)y roclv decay. However, if abundant 

 organic matter is present in the soil layer on the weathered mantle, conditions 

 are favorable for the reduction of the ferric oxide to ferrous oxide, and the 

 formation of carl-jon dioxide. The presence of decaying organic matter in 

 the soil thus not only reduces the ferric oxide but furnishes at the same time,, 

 in the presence of water, a solvent for the ferrous oxide which may be taken 

 into solution and carried downward to join the underground water circula- 

 tion or to be reprecipitated as ferric oxide l)efore reaching the belt of satura- 

 tion. If the Ijelt of saturation be near the surface a proportionately 

 large amount of the ferrous iron Avill join the underground waters, in 

 which it may be transported to places where the conditions are favorable 

 for oxidation and precipitation. Under the physiographic conditions 

 outlined above, such places would be found along the overground drainage 

 lines where the underground seepage waters mingled with the oxygenated 

 surface waters. If the surface waters were sluggish or ponded in marshes 

 the precipitated ferric oxide would sink and become mingled with the mud 

 and sand of the bottom. 



Some such conditions are believed to have prevailed in the vicinity of 

 Spring \"alley when the Gilman, Cady, and other smaller but similar deposits 

 Avere formed. The accumulation of the ores in favorable places along the 

 stream courses was terminated by an uplift of the land and the resumption: 

 of downward cutting by the streams. T]je valleys were rapidly deepened 

 and the water table Avas correspondingly lowered in relation to the land, 

 surface, while at the same time the underground circulation was invigorated. 

 Where ore deposits had formed in the old valley bottoms they were cut, 

 through and ])artially removed. The Gilman, Cady, and other deposit?; 

 were formerly of much greater extent, just how much greater can not be 

 tletermined, but their present positions on the upper slopes of the valley 

 sides would seem to indicate that the parts which have been removed are 

 greater than those that remain. The topographic relations shown in the 

 Gilman and Cady deposits are common to all of the other smaller deposits 

 in the Spring Valley area. Says Mr. Wallace Foote, "most of the ore in 

 this country is on the sides of hills or on the very edge of the top." Those 

 deposits which were formed nearest the divides in the old vallej's would be 

 now at greater elevations than those formed further down stream had none 

 of the deposits been moved from the positions in which they originated, 

 and deposits formed in different vallej's would probably be at somewhat 

 different elevations, but the chief cause for the varying altitudes of the 

 deposits is probably to be found in the tendency of the ore beds to creep 

 downward over the steej) valley sides toward the stream channels. This 

 is well illustrated in the Gilman deposit where the ore has slumped down- 

 ward at least an amount sufficient to completely bury a rock cliff which was 

 formerly exposed to the air and formed part of the valley side. In this 

 downhill creep the ore has become intermixed with residuum as is shown 

 by the presence in the ore bed of numerous more or less decomposed chert 

 fragments derived from the decay of the underlying limestone in which 

 chert is abundant. The remarkable thickness of the ore in the shaft at the 

 Cady mine and its relation to the face of limestone which was encountered 

 at a depth of SO feet and followed downward at an angle of 6D degrees for 

 40 feet is accounted for in the same manner. The ore has slumped down- 

 ward over the hill side and completely buried a limestine cliff. The history 

 of a deposit of this character is indicated in figures 3 and 4. 



