walcott.] CORRELATION. 431 



between the Laurentian and the Huronian, between the Huronian and 

 the Keweeuawan, where the two series of rocks are present, and be- 

 tween the Keweeuawan and the Cambrian if these two occur iu the same 

 area. 



He next takes up the use of unconformities in establishing general 

 relations. Under this heading we quote as follows : 



Having established the general order of succession and the grander groups of the 

 strata for a given geological province, the question which arises next in order is how 

 to correlate these divisions with those of other geological provinces, and more partic- 

 ularly with the established divisions of the general geological column. 



It will take but little consideration of the causes which have been at work in the 

 production of such unconformities as have been above cited to make us realize that 

 such breaks as these must be widespread in their influence. Great unconformities, 

 in which the strata below the unconformity have been subjected to folding, mark 

 periods of orographic movements which will, in general, have been extensive some- 

 what in proportion to the intensity of the folding process. 1 



It is well known, indeed, that some of the greater physical breaks in the strata of 

 one continent may have their parallels among the strata of another continent. The 

 interruption between the Paleozoic and the Mesozoic is intercontinental, if not world 

 wide. Equally extensive is the great break between the Mesozoic and the Cenozoic. 

 Each of these physical breaks corresponds to au immense change in life conditions. 

 But, if we may judge from structural relations, from the amount of intervening 

 denudation, and from the rank already attained by the life of the lowestofthe fossil- 

 iferous Cambrian formations, neither of these great breaks corresponds in length of 

 time interval to the break between the lowest of the distinctly fossiliferous forma- 

 tions and the youngest of those beneath it.' 2 



The great structural breaks furnish, at times at least, a more trustworthy guide. 

 If, for instance, we make a comparison of the succession of Cambrian and pre- 

 Cambrian strata in the Lake Superior region with the succession revealed in the 

 depths of the Grand Canon of the Colorado, we find some singular correspondences 

 between the structural breaks of the two successions. In each one of these regions 

 the Potsdam sandstone is strongly characterized by its well known fauna. In each 

 region the sandstone traverses the edges of a gently bowed but deeply denuded 

 formation composed of a great thickness of detrital and eruptive materials. Below 

 these formations in each region, and separated from them in each case by one of the 

 strongest of unconformities, is a great quartzitic series, while below this series 

 again in the Lake Superior region, and separated from it by still another of the most 

 notable of unconformities, is the great Laurentian gneissic series. That this series 

 also exists in the base of the Grand Cafion we have good reason to believe. 



Such a striking similarity in succession as this certainly most strongly suggests the 

 conclusions that the physical breaks of the two regions were coincident in point of 

 time (that is, that the mountain-making movement which preceded the deposition 

 of the Potsdam sandstone of the Lake Superior country corresponded with that which 

 preceded the Tonto sandstone of the Grand Canon region) ; that the mountain-mak- 

 ing period* preceding the deposition of the Grand Canon series of the Grand Canon 

 district and of the Keweenaw series of the Lake Superior region were equally syn. 

 chronous; and, finally, that the Grand Canon series is, in general, the equivalent of 

 the Keweenaw series, while the pre-Graud Canon rocks are the equivalent of the 

 Lake Superior Huronian. 



Of course such correlations as these should be made, for the present at least, some- 

 what provisionally; but, on the other hand, they must be taken as of much greater 

 value than correlations between distant formations established on feeble fossil 

 evidence. 3 



» 0f>. cit., p. 443, »0p.cit.,p.444. 3 0p.cit.,p.445. 



