238 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
extensive exposures of the formation. So true is this, that it is quite difficult to dis- 
tinguish the true bedding planes. 
* * * ¥* ¥ * So * * * 
In addition to these vertical planes of division, which are generally quite smooth and 
uniform, but not persistent to great depths, there is another prominent set which are 
much less smooth, but much more persistent and constant in direction. The surface of 
the layers separated by these joints is nearly uniform to the general view, but in detail 
is slightly uneven and undulatory, as though the separation took place not through the 
fracture of a homogeneous rock, but by separation along a natural division plane, 
These planes are usually separated by several feet. They are confidently believed to 
represent the dip of the igneous beds. It is not presumed that all of the layers so 
formed represent separate overflows of molten material, much less distinct periods of 
eruption; but that in the flowage and outspreading of the igneous matter, a somewhat 
parallel arrangement of the not perfectly homogeneous substance took place, giving 
rise to an obscure pseudo-stratification, sufficient to influence the jointing that subse- 
quently took place. At the same time, the fact that the beds at different horizons pre- 
sent different textures, and, in a subordinate degree, different mineralogical composi- 
tion, would seem to favor the belief that the several hundred feet of the formation 
exposed in the vicinity of Saint Croix Falls, represent a considerable number of dis- 
tinet but closely successive overflows; all, perhaps, belonging to one great period of 
eruption. The latter statement seems to be demanded by the lithological similarity of 
the rock, the slight distinction between the beds, and the absence of detrital deposits 
between them. Notwithstanding their obscurity, however, the beds give to the out- 
crops the distinctive step-like or trappean contour that has been previously described 
and figured. This is best seen in the exposures about one mile east of Saint Croix 
Falls (N. E. 4 of Sec. 29, and N. W. 4 of Sec. 28, T. 34, R. 18 W.), where the inclined 
ledges follow each other with much regularity and persistence, giving to the profile of 
the cross-section a serrate outline, notwithstanding the fact that the glacial agencies 
acting from the northwest tended to plane down the edges of the beds. 
It is upon the persistence of these inclined ledges, taken in connection with par- 
allel lithological belts, that our determination of the dip, a matter of some theoretical 
interest at this extremity of the formation, is mainly based. The average of a large 
number of guarded observations gives a dip of about 15° W. by S. This inclination 
to the south of west is quite an interesting fact, however it may be interpreted. To 
the writer it seems to signify, taken in connection with other observations, that the 
trough of the Lake Superior synclinal, at this western extremity, curves rapidly south- 
ward, and is connected, over a sort of saddle-back anticlinal, with the broad strati- 
graphical basin that stretches southward into Minnesota; and that the igneous beds 
overlap this figurative saddle-back, so as, on their margin, to really lie in the southern 
or Mississippi basin. This low anticlinal is supposed to lie a little north of Saint Croix 
Falls, and to be the low, flattened extremity of the Laurentian and Huronian heights 
that lie to the eastward—the saddle-bow of our illustration. 
The unconformity shown by Messrs. Sweet,’ Strong,” and Chamberlin? 
to obtain in the Saint Croix Valley between the fossiliferous Cambrian 
1 Transactions of the Wisconsin Academy of Sciences, Arts, and Letters, Vol. IT, p. 40. 
2Geology of Wisconsin, Vol. III, Part VI. 
