GENERAL GEOLOGY OF THE DISTRICT. 459 



fault witli a southward throw, l)v means of which a section of tlie Southern 

 C<)in})hix has been set off from the corresponding- rocks east and west. But 

 this assumption of l)oth unusual erosion and faultino- has no positive facts in 

 its favor, and it seems, in tlie lack of definite evidence, that it is probable that 

 the Penokee series is continuous and that the g-reenstones adjacent to the 

 granite in this area are intrusive within them, rather than a part of the 

 Keweenawan basement flow. 



About half a mile west of the east range line of R. 4 W., Wisconsin, 

 numerous exposures of the iron formation are again found. The known 

 thickness of the Penokee series is here about half a mile, while it may be 

 considerably thicker. At the center of Sec. IS, T. 44 X., R. 3 ^Y., Wiscon- 

 sin, the series is at least thi'ee-fourths of a mile wide. Just east of English 

 lake this width has become more than a mile. This widening- of tlie series 

 g-oes on continuously until Tylers fork is reached, T. 45 N., R. 1 W., Wis- 

 conshi, where its maximum thickness is found. In passing- ea.stward from 

 this stream the -series gradually narrows with great uniforniit\' until a short 

 distance west of Sunday lake, in the we.st part of T. 47 N , R. 45 W., Mich- 

 ig-an, where the Upper .slates have disappeared. From this point to the 

 middle of T. 44 W., Michigan, the overlying Keweennw rocks are in direct 

 contact with the Iron-bearing formation. At one place'just east of Sunday 

 lake this formation can not be ex])osed for more than a third of its width. 

 Through townships 44 W., 43 W., and 42 W., Michigan, the greenstone 

 ridge constituting the Ijasement of the Keweenaw series passes nearly in 

 an east and west direction, with the exception of 1.^ miles in the east part 

 of T. 47 N., R. 44 W., Michigan, and here the discrejiancv is explaincil (pp. 

 424, 425) as being probablv due to a fault. In these town.ships tlic immedi- 

 ately underlying rock is a mingled fragmental and nonfragmental one 

 which, as explained (p. 431), is probably the eipiivalent of the upper part 

 of the Iron-bearing- member to the west. 



That the Penokee series could have been deposited to a thickness of 

 about 14,000 feet at Tylers fork, and at other points l)e only 1,000 feet 

 thick, or entirely absent, is not at all probable. This is especially true, as 

 it is the lower 1,000 or 1,500 feet of the series AA-hich is found whenever 

 any part is present. As already explained, the persisteni-e of the Quartz- 



