3i8 



H. C. COOKE 



and was thus able to observe directly that the width of the flattened 

 strata along the axis was slight, perhaps loo feet or thereabouts. 

 It is clear therefore that the minimum amount of erosion necessary 

 to produce such a condition, represented by CC in Figure 4, can be 

 determined by measuring the thickness of the strata by the usual 

 methods. All that is necessary for such a determination is a 

 knowledge of the position of the axis of an anticline and the next 

 adjacent syncline, which may be observed directly or calculated 

 from the position of some recognizable horizon. 



L -B 



Fig. 4.— Diagram illustrating method of calculating minimum amount of erosion 

 at the base of the Cobalt series. 



It need scarcely be emphasized that such a determination of 

 the amount of erosion is no more than a minimum, placing the 

 erosion surface in the position A A' (Fig. 4). It gives no hint of 

 the additional thicknesses of rock removed in the formation of any 

 lower erosion surface BB' . 



In Boston and Otto townships a recognizable horizon for pur- 

 poses of measurement is afforded by a band of well-stratified tuffs 

 approximately 1,300 feet in width which are here interbanded 

 with the basalt flows (Fig. 5) This band was determined, by 

 methods described/ to have the north side as the upper. In the- 



^Jour. Geol, Vol. XXVII (1919), P- 75- 



