THE EASTERN AEEA. 425 



gested b)' the position of the great mass of basic eruptives in Sees. 2C and 27, 

 T. 47 N., R. 44 W., Michigan. Tliese exposures are all essentially the same 

 kind (if rocks, coarse grained diabases, ai)proacliing gabbro. Such rocks 

 would commonly be regarded by petrographers as deep seated ones, which 

 have now reached the surface by erosion. They are remarkably similar to 

 the massive eruptives in Sees lo, Ki, and 23, T. 47 N., R. 44 W., Michigan, 

 and they are probably contemporaneous in time, if not actually connected 

 with these rocks. They now lie at the same apparent horizon as the large 

 exposures of typical greenstone-conglomerates just east of them, which are 

 surface accumulations. If the discrepancy between the locations of the iron 

 formation in Sec. 25, T. 47 N., R. 44 W., Michigan, and those in Sec. 23 

 can be taken as a guide to the amount of throw of the supposed faidt, it 

 woidd reach several thousand feet. If such a fault occurs, it woidd, to tlie 

 extent of its throw, lessen the thickness of the formations belons'ino- to the 

 eastern area, as will readily be seen b}' examining PI. xiii. Supposing no 

 fault to exist, the maximum thickness of tlie series, if the succession is a 

 simple one, nmstbe about 11,000 feet in the exti-eme eastern part of T. 47 N., 

 R. 44 W., Michigan. If we suppose a fault to exist with a throw of 3,000 

 feet, this Avould reduce the maxinnun width to 8,000 feet. East and west 

 of this locality the thickness rapidly lessens. At the center of T. 47 N., 

 R. 43 W., Michigan, 3 miles east, the sui-face width of the series is about 

 1 mile, which represents a thickness of ab(iut 4,500 feet. In passing far- 

 ther east, the series continues to narrow until it is almost entirely over- 

 lapped by the Eastern sandstone in Sec. 28, T. 47 N., R. 42 W., Michigan. 

 To the west the series also rapidly narrows until it joins the normal Peno- 

 kee area. 



The rapid vaination in thickness of the Eastern area may be urged in 

 opposition to the above explanation of its great thickness and of the bow- 

 ing of the iron-bearing formation to the south. In answer it may be 

 said that the distance from the easternmost to the westernmost exposure 

 of the greenstone-conglomerate is fully 7 miles, and the evidence of dis- 

 turbance, as shown by the mingling of fragmental and nonfragmental 

 material, extends to twice this distance. Also, when it is considered that 

 the greater part of this thickness is due to greenstone-conglomerates, which 



