454 



SCIENCE. 



[Vol. I., No. 16. 



one hag seen the formation in its entirety; and only 

 one investigator lias approached to ai general famil- 

 iarity with it by personal study, and his more coinpre- 

 hensive results are not yet before tlie public. 1 have 

 even liesitated on this account to offer this summary, 

 having myself visited only seven of the significant 

 districts outside of Wisconsin, witli the investiga- 

 tions within wliich I have, .of course, been intimately 

 familiar, as also witlithe results of Prl)fe^sor Irving's 

 more extended studies, which are herein somewhat 

 drawn upon. 



Brevity requires the omission of citations and au- 

 thorities in the main. 



The general slratigraphical facts wliich are not 

 open to reasonable question are these: 1. Around 

 the edges ot the great depression occupied by Lake 

 Superior lies an immense series of interleaved igneous 

 and detrital beds, dipping inward toward a synclinal 

 axis, lying mainly beneath the lake, but stretching 

 landward across north-western Wisconsin; 2. Both 

 within and without this basin are horizontal series 

 of sandstones, each of which is traceable into contact 

 with the dipping series at a few points, and into 

 approximate junction at several others. The hori- 

 zontal sandstone on the outside contains primordial 

 fossils, and has long been known as Potsdam. The 

 horizontal sandstones within the trough, unfortu- 

 nately, have not yet yielded fossils of any positive 

 character. Some of these are so situated that they 

 might be supposed to be portions of the synclinal fold, 

 but the greater part are not so placed as to admit of 

 this interpretation. 



Now, those who advocate the distinctness of the 

 Keweenaw series maintain that the great tilted group 

 of interbedded igneous and detrital rocks which con- 

 stitute the copper-bearing formation belongs to an 

 entirely different age from the horizontal sandstones 

 without, and from most, but not all, those within. 

 They offer, among other considerations, the following 

 classes of evidence in support of their view: — 



1. First and weakest, the general siralv/raphical 

 relations above indicnted. — These afford at least a 

 presumption of distinctness. Tills admits of easy 

 verbal objection, and to those personally unfamiliar 

 with the tout ennemble of tlie j^roblem and its data, 

 and with the methods Nature habitually pursues in 

 distinction from those she might be imagined to 

 pursue, can have but little force; but e.xperienced 

 stratigraphlsts will appreciate the fact, that great dif- 

 ferences in the altitudes of closely associated strata, 

 especially if otlierwise differentiated, are usually indic- 

 ative of differences in age, and that definite evidence 

 of unity is required to justify the somewhat violent 

 dynamics necessary to otherwise explain these diverse 

 attitudes. This is especially true when the surround- 

 ing I'egion is altogether devoid of evidence of disturb- 

 ance (luring the supposed period of disruption. Not 

 only in the immediate vicinity, but throughout the 

 interior, there is an absence of evidence of more than 

 the gentlest oscillations in the recognized primordial 

 strata; while the Keweenaw series suffered a depres- 

 sion of more miles than it would seem judicious to 

 estimate here, and embraces one of the most stu- 

 pendous series of eruptions known to early geologi- 

 cal history. Upon this argument, l)elng a general 

 one, we do not much insist. It gains force, however. 

 In connection with the following ijoints, and gives 

 especial significance to the next. 



2. Differences in Ihicknexs. — The recognized Pots- 

 dam strata in the adjacent region have been pene- 

 trated at numerous points by artesian wells, and are 

 only rarely found to reach a thousand feet in 

 depth. On the other hand, the thickness of the Ke- 



weenaw series is so enormous as to have led to a 

 studied watchfulness for possible sources of error of 

 estimate. Unless faults be assumed where there is 

 no proof of them, the maximum thickness must be 

 upwards of forty thousand feet, of which alxiut fif- 

 teen thousand feet are detrital. Without insisting 

 in a controverted matter, that this estimate may not 

 be too high, owing to undiscovered faults, it remains 

 that an enormous difference is absolutely demon- 

 strable. Now, this great difference means something 

 in the mere matter of accumulalion, but great stress 

 is not laid upon this. Plausible, but really inap- 

 plicable, answers readily suggest themselves. If, 

 however, it is insisted that the igneous eruptions 

 furnished exceptional conditions for rapid accumula- 

 tion, it will be freely granted, and even urged : but the 

 great mass of the detrital beds were formed after the 

 eruptions had ceased; and, besides, the fossiliferous 

 Potsdam strata lie against the same rocks in the St. 

 Croix region, and, if contemporaneous, should have 

 been likewise favored in accumulation. 



But whatever this incongruity of thickness signi- 

 fies in the question of deposition, it is at least impor- 

 tant in the interpretation of the discordant attitudes 

 of the strata, and the adjudication of approximately 

 observed, but not actually visible, unconformities. We 

 hold that to be a violent structural hypothesis which 

 assumes that portions of the same unmetamorphosed 

 series are tilled at high angles, while, within a dis- 

 tance much less than the thickness of the formation, 

 other portions lie undisturbed. That this extraor- 

 dinary phenomenon should be several timi's repeated, 

 in a region not otherwise characterized by more than 

 broad open folds, seems to us incredible. 



3. Differences in constitution. — The sandstones of 

 the Keweenaw series are largely composed of grains 

 of various .siiicfiies derived from igneous rocks; while 

 the Potsdam, within as well as without the basin, is 

 mainly quartzose, as shown by the investigations of 

 Irving and Sweet. The former are manifestly <lerived, 

 as maintained by these writers in common with others 

 on both sides of the question, immediately from the 

 igneous series, with relatively little wear or assort- 

 ment. The latter are thought to have had wider 

 sources, and to have been subjected to more erosion 

 and winnowing; for even where in the vicinity of 

 the igneous series they are still notably quartzose. 



4. Uiicoiiforniit!/. — While every unconformity has 

 a significance, only those are urged in this relation- 

 ship which seem to us to testify directly to the fact 

 ot a tilting of the great copper-bearing beds before 

 the Potsdam sands were laid down upon ami against 

 their upturned edges. The cases of unconformity 

 may be grouped in three classes: a, those actually 

 observed; b, those in which the contact, though ob- 

 served, is complicateil with disruption; ami, c, those 

 in which the immediate junction is concealed, and 

 the evidence is only approximate. 



a. Of the first class are those of the St. Croix dis- 

 trict, substantiated by the independent observations 

 of Sweet in 187.5, Strong in 187G and 1877, WInchell 

 at one or more dates unknown to me, and myself 

 in 1S76, 1879, and ISSO. There are also liere several 

 cases of approximately visible junctions beside those 

 actually seen. To us, the facts — which manifestly 

 cannot be properly described here, but which are in 

 a measure set forth in the Wisconsin publications — 

 teach explicitly that the copper-bearing bi-ds were not 

 only formed, but uplifte<l and extensively worn into 

 hills and valleys, before the Potsdam saiuls were laid 

 down against and upon them. The full force of the 

 evidence presented by this region can only be felt 

 when a just appreciation of the facts is acquired, 



