Quartzytes a7id Quartz-Rocks. 185 



to one group, the difference in aspect being supposed to be 

 solely due to local circumstances. Logan, however, showed 

 that there must be a considerable difference in age between 

 the Potsdam and the underl^dng Huronian quartzytes at St. 

 Marie River, while since then, Irvine, Van Hise, Lawson, 

 and others, have traced out profound unconformabilities which 

 prove that quartzytes, formerly supposed to be all portions of 

 the Potsdam, may belong to strata on different geological 

 horizons, and also to distinct terranes. 



In Central Wisconsin, in the neighbourhood of Baraboo, the 

 Potsdams seem to lie horizontally on the Huronian quartzytes, 

 but seventy-five miles to the north, near Stevens, they cap 

 Laurentian quartzyte and gneiss. More often than not, the 

 Potsdams appear to lie conformably on the Huronian quart- 

 zytes and the older Laurentians, but when worked out it is 

 not so, as pointed out by R. D. Irvine in his exhaustive 

 report on the " Early Cambrian and Pre-Cambrian Forma- 

 tions of the Lake Superior Regions." 



To give all that can be learned from the results of the work 

 in the Lake Superior regions, would occupy too much space ; 

 we will therefore only refer to the Marquette and Meno- 

 minee sections, as in these regions are exhibited the pheno- 

 menon that the ofiicers of the staff of the Geological Survey 

 of Ireland are far from realising. In both of these regions 

 (ibid. pp. 434 et seq.^, the newer terranes and the associated 

 older schist series have, by excessive thrusting from the south- 

 west, been so sharply folded up, that now the newer strata 

 occur as long, narrow, lenticular tracts, with a nearly identical 

 strike to that of the older rocks, and apparently with similar 

 dips. Mining operations have, however, proved the latter ap- 

 pearance to be erroneous.' 



The distinctions between the old and later rocks are, firstly, 

 in places the lower beds of the latter are coiiglomeratic, containing 

 pebbles of the older rocks. This, however, is a proof that can be 

 easily misconceived, as there are various ways to account for 

 the presence of the pebbles, as illustrated in the writings of 

 the early American investigators and the theories ot the 

 British geologists. Secondly, as a general rule, the olde? rocks 

 are more schistose thaji the later o?ies. This, however, is also 

 an unsatisfactory proof, because at places in the latter, such 

 as at an acute end of a fold, also adjoining up thrust planes, 

 the schistosity may be excessive. Thirdly, in the older rocks, 

 there are granyte and other intrusive rock-veins, also fault lines, 

 that come up to but never extend beyond the boundary of the newer 

 rocks. These are to me uncontestable proofs of an uncon- 

 formabilit}^, yet there are eminent English and Scotch geolo- 

 gists who seem to ignore such evidence. 



The ignorance of geologists as to all unconformabilities, 



* A home example is the "stage-lode," Bonmalion, Co. Waterford, — 

 "Geology of Ireland," p. 28. 



