430 THE CAMBRIAN. [bull. 81. 



Huronian unconformity on the north shore of Lake Huron, the Pots- 

 dam-Keweenaw unconformity, the Laurentian-Huronian unconformity 

 and others of the Lake Superior region. This is followed by a section 

 on the use of unconformities in classification. 1 In this the use of un- 

 conformities in defining the grander groups of strata and in correlating 

 the formations of a single geologic basin are discussed. Under the 

 first, he says : 



Returning to our problem as originally stated, we have next to consider how far 

 uuconformaties may be made use of in defining the grander groups of strata in a 

 region in which the succession of these strata has been determined. Further argu- 

 ment than that already given in discussing the nature and kinds of unconformity is 

 hardly necessary to show that genuine unconformities, indicative of great lapses of 

 time between the periods in which the strata on either side of the break in each 

 case were respectively made, are of prime value and importance in determining the 

 limits of the grander groups of the geological formations, whatever use we may 

 make of paleontological and lithological characters in determining subordinate 

 divisions. Such structures as the greater ones of the true unconformities, above con- 

 sidered, indicate lapses of time great enough to cover extended periods of mountain- 

 making, always a slow process, and also great periods of denudation or exposure to 

 the atmospheric agencies. It is hardly possible for us to compare such time gaps 

 with the time necessary for the formation of definite thicknesses of strata, but the 

 thicknesses of strata corresponding to such breaks must surely always be very great. 



If we take for instance the gap indicated by the relations of the Potsdam sand- 

 stone to the ancient gneissic formation of the northwest, as above illustrated by a 

 number of examples, we find that it was long enough to cover not merely one period 

 of rock alteration, orographic movement, and land surface exposure, but three such 

 periods, between which were times long enough for the accumulation of thicknesses of 

 strata aggregating over 60,000 feet, a thickness exceeding that of the entire Paleozoic 

 series in its greatest development in the Appalachian region. It is true that a portion 

 of this 60,000 feet is made up of volcanic materials, chiefly lava flows. The accumu- 

 lation of such materials may, it is true, have been more rapid than is the case with 

 ordinary fragmental deposits ; but the accumulation of such a mass of eruptive 

 material must have occupied at least a considerable lapse of time; while more than 

 one-half of the 60,000 feet, or an amount approximating the maximum Paleozoic ac- 

 cumulations in the Appalachian region, is made up of genuine detrital deposits. When 

 we consider that in addition to the time necessary for the accumulation of this mass 

 of sediment there intervened between the Potsdam and the gneisses three periods of 

 rock alteration, mountain-making, and complete mountain removal, it becomes plain 

 that the time gap indicated by this unconformity must have exceeded the entire Paleo- 

 zoic era. More probably, indeed, it equalled all later geological time. Each one of 

 the three unconformities mentioned must of course have required a shorter period 

 than this greater gap, but in each case the relations of the several formations are 

 such as to indicate periods of time only comparable to the periods necessary for 

 the accumulation of one of the great geological groups. 2 



The use of unconformities in correlating the formations of a single 

 geologic basin, where the exposures of the formations are not continu- 

 ous, is illustrated by a tabulation showing the general succession of 

 formations in various districts from the northern end of Lake Huron to 

 Dakota and from central Wisconsin to the north shore of Lake Superior. 

 This table shows that in various localities there is a great unconformity 



' Op. Cit., p. 438, * Op. cit., pp. 438, 439, 



