424 THE PEXOKT-:]<: iiiON bearing series. 



volcanic iiiatcrial niay go on witli rapidity much greater than tliat of ordi- 

 nary sedimentation. For sucli material to accumulate to a deptli of 7,000 or 

 8,000 feet while a ivw luuidred feet of the Iron-l)earing member to the west- 

 ward ^\'ere formint;' is not contrar\' to known o-eolog'ical facts. The nortliern 

 • boundary of the Eastern area, throughout the district is approximately east 

 and west, with no greater variation than in the main area. The increased 

 thickness of the series is given space by rapid bowing to the southward of 

 the underlying complex. This southern bowing is explicable as resulting 

 from a continuous sinking, which was contemporaneous witli and caused 

 by the accumulation of great quantities of ejected material. This Ixiwing 

 is simply the well known geological plienomenon of the inwai-d sinking 

 of the strata al)out a crater when l)urdened witli a mountain mass of vol- 

 canic products. 



The sudden bowino- of the iron-bearing- belt of the series to the south 

 may be due in pai-t to a fiiult. Upon the whole, it seems not uidikel)" 

 that such a fault has combined with the extravasation of lava and tuff 

 to produce the present condition of aftairs. The strike of the various 

 exposures found in T. 47 N., R. 44 W., and 1\. 43 W., Michigan, shows 

 distinctly that a bowing to the south has, occurred. This is e^•idcuced 

 by the strike of the easternmost exposure of the continuious Iron-lxMring 

 member in Sees. IC and 17, T. 47 N., R. 44 \V., Michigan, as avcII as the 

 strike of the exposures in T. 47 N., R. 43 W., Michigan. It is, however, 

 noticeable that the iron formation rocks in the N\V. \ of Sec. 30 strike about 

 E. 20° S. The i-ause of this anomaly is not known. The greatest appar- 

 ent discordance between the various exposures of the iron formation 

 is between that in the west part of Sec. 23, T. 47 N., R. 44 W., and 

 that in the southwest j)art of Sec. 25. It is also to be observed that 

 the l)ase of the Keweenaw series, which in '\\ 47 N., R. 42 W. and 48 

 W., Michigan, has a nearly east and we.st trend, and which west of the 

 S. ^ post of Sec. 11, T. 47 N., R. 44 W., Michigan, also lias a very uniform 

 east and west trend, between these two points differs a lialf mile in position. 

 Tims it is probable that if a fault exists it runs from near the smithwest 

 corner of Sec. 25 to a sliort distance west of the E. \ post of Sec. 13, T. 47 

 N., R. 44 W., Michigan. The existence of such a fault is still further sug- 



