470 TERENCE T. QUIRKE 
feet over large areas.* And in the Espanola’ area there is a prob- 
able difference of erosion of more than 5,000 feet within ro miles. 
It is largely a matter of opinion as to how important such an 
unconformity should be considered, but it is surely not inconspicu- 
ous. Erosion intervals of thousands of feet in Paleozoic series are 
considered notable, and they should not be undervalued in pre- 
Cambrian series. However, in fairness to all it must be added 
that there are places where the unconformity appears to be slight; 
generally only by tracing it over large areas, as Collins has done, 
can the observer recognize its true greatness. Furthermore, in 
comparison with the great unconformities beneath and above the 
Huronian formations the Bruce-Cobalt unconformity is much less 
conspicuous. ‘This at least may be said, the unconformity locally 
is inconspicuous but nevertheless important. 
Second, in saying that the Bruce series, the lower Huronian 
series of Ontario, locally contains tillites, Steidtmann appears to 
follow Coleman (1915).3 The fact is that the tillites are charac- 
teristic only of the upper group, the Cobalt series, or specifically 
the Gowganda formation of Collins.4 The glacial origin of at 
least part of the Cobalt conglomerate is held by Wilson (1913),5 
Collins (1914),° and by Coleman himself first (1907)? and last 
(1920). The Bruce conglomerates in certain phases are distinctly 
different in character from the Cobalt tillite. They lack the thinly 
laminated slates and the well-bedded slate layers carrying scattered 
= W. H. Collins, Canada Geol. Survey, Museum Bull. No. 8 (1914), p. 21. 
27, T. Quirke, Canada Geol. Survey, Mem. No. 102 (1917), p. 42. Apparently 
through errors in copying, the summary of Quirke’s classification on p. 657 of Steidt- 
mann’s paper differs considerably from the work summarized, both by omissions and 
by faulty arrangement. See Canada Geol. Survey, Mem. No. 102, pp. 6 and 7. 
3 Quoted by Steidtmann, op. cit., pp. 646, 648. 
4W. H. Collins, Canada Geol. Survey, Mem. No. 95 (1917), p. 10. 
5 Morley E. Wilson, Jour. Geol., Vol. XXI (1913), pp. 121-41, and Canada Geol. 
Survey, Mem. No. 17 (1912). Quoted by Steidtmann, op. cit., p. 658. 
6 W. H. Collins, Congrés géologique international, X1Ith Session (1914), pp. 399- 
407. Quoted by Steidtmann, op. cit., p. 650. 
7A. P. Coleman, Am. Jour. Sci., Vol. XXIII (1907), pp. 187-92; Jour. Geol., 
Vol. XVI (1908), pp. 149-58; Bull. Geol. Soc. Am., Vol. XIX (1908), pp. 347-66. 
8 A. P. Coleman, Economic Geology, Vol. XV (1920), No. 6, pp. 539-41. 
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