STRUCTURAL GEOLOGY. 47 



Age of the Animikie.l 



loidal (?) trap (No. 1847) lies on the Animikie at the most southerly of the Lucille 

 islands. This is supposed to be near the top of the Animikie. (2) The record of 

 the deep well drilled at the Short Line park, near Duluth, shows a series of trap 

 layers resembling those seen at Duluth below the quartzose conglomerate exposed 

 in the St. Louis valley near Fond du Lac. This conglomerate is supposed to be the 

 western representative of the Puckwunge conglomerate. (3) In the vicinity of 

 Beaver bay, where the Beaver Bay diabase is charged with masses of feldspar rock, 

 there is seen to be a loose red conglomerate and a diabase basalt below the Beaver 

 Bay diabase. If there be equivalence of date between the Beaver Bay diabase and 

 the parent mass of gabbro, as supposed, then there must have been at least one and 

 perhaps several surface lava flows anterior to the outbreak of the gabbro revolution. 

 This superposition is seen at the bite of the little bay (Two Harbor bay of the annual 

 reports) sec. 12, T. 54-9, where the lower portion of the great diabase is a pudding- 

 stone of feldspar masses. All this evidence, however, is still inconclusive and the 

 nature of the top of the Animikie remains an unknown quantity. It may have 

 blended gradually with volcanic ash and occasional lava sheets, but was terminated 

 by the great revolution which gave origin to the gabbro and the red rock. The first 

 recognizable and fixed datum, in the form of a clastic stratum, after the Animikie, 

 is the Puckwunge conglomerate, and that is also posterior to at least some of the 

 great dikes of Grand Portage, to the red rock and to the great gabbro mass. 



The age of the Animikie is considered to be that of the Lower Cambian. It may 

 be below the horizon of Olenellus, which sometimes restricts the downward extension 

 of the Lower Cambrian, but as it is followed above by the sandstones which graduate 

 upward into the Upper Cambrian, as seen on the south side of lake Superior, and in 

 the valley of the St. Croix, it is at least in the proper place for the Lower Cambrian. 

 According to the conclusion reached by Mr. Spurr that the iron ore of the Mesabi is 

 derived from a glauconitic sandstone, there is reason to presume that large quantities 

 of foraminiferal organisms once lived in the Animikie ocean. In the St. John group 

 of New Brunswick, Mr. W. D. Matthew has shown* the existence of foraminiferal 

 forms associated with noticeable amounts of iron ore. In the Animikie in the 

 Thunder Bay region, Mr. G. F. Matthew has also described a Taonurus-like impression 

 which he has named Medusichnites.f The Canadian geologists uniformly refer the 

 Animikie of the Thunder Bay region to the Lower Cambrian. 



Perhaps the strongest indication of the non-Archean age of the Animikie lies 

 in the non-conformity at its base. All earlier strata are highly tilted, and usually 

 nearly to vertically. The Animikie is rarely tilted as much as forty-five degrees 

 from horizontality, and over large areas it is nearly level, lying on the vertical strata 



"Trans. N. Y. Acad. Sci., vol. xii, pp. 108-120, 1893. 



t Trans. Roy. Sac., Canada, vol. viii, Sec. iv, p. 143, 1800. 



