THE BLACK HILLS CRYSTALLINES PROBABLY ALGONKIAN. 239 



It is also clear that to the Algonkiau period belong the series which have 

 been designated as Hnronian by the Michigan and Wisconsin geologists, 

 although by no means covering all the rocks here included by the Canadian 

 survey. The question immediately arises whether the modified elastics of 

 the Black Hills can be correlated with the iron-bearing series of Lake Supe- 

 rior. This question . cannot be positively answered. . Newton, from the 

 uncertain data which at the time of his study was available as to the nature 

 of these Lake Superior rocks, thought it probable that his slate series was 

 their equivalent.* Crosby and Carpenter regard these slates as rather the 

 equivalent of certain of the Taconic schists. As to the probable truth of 

 the latter correlation I have no opinion, because I have no personal famil- 

 iarity with the Taconic rocks and do not know whether any of them can 

 reasonably be regarded as equivalent to the Lake Superior iron-bearing series. 

 However, the case to-day for placing the Black Hills slates and schists as 

 the possible equivalent of these series is very much stronger than when 

 Newton wrote. Belonging to the Animikie, Penokee, and Marquette series 

 are great thicknesses of mica-slates and mica-schists. These micaceous rocks 

 are certainly of fragmental origin and have a genesis similar to those of the 

 Black Hills. Like them, they are staurolitic and garnetiferous in certain 

 cases. The thick beds of nearly pure quartzite and quartzose conglomerate 

 which occur in the Black Hills are parallelized by quartzites and conglom- 

 erates in the Marquette and Penokee areas. Much of the iron-bearing 

 formations of the Lake Superior region have been shown not to be mechan- 

 ical sediments, but rather chemical or organic sediments which by subsequent 

 alterations have been changed into the various forms now found. f In the 

 Black Hills of Dakota occur considerable beds of rock so like those of the 

 iron formations of Lake Superior that they cau hardly be distinguished from 

 them. In the Lake Superior region important beds of iron ore are known 

 in these formations. Such are not yet known to occur in the hills. In the 

 Lake Superior region are vast quantities of basic eruptives which occur in 

 dikes, bosses, and intrusive beds in the fragmental series; similar rocks in 

 similar relative position are again found in the Black Hills. The chief 

 lithological difference between the two regions is the presence in the Black 

 Hills of large masses of granite. The only known parallel to this occurrence 

 in the Lake Superior iron-bearing series is found in one or two unimportant 

 dikes. 



This lithological correspondence between the Black Hills rocks and certain 

 of the Lake Superior iron-bearing series is truly remarkable. In cases in 

 which a set of similar conformable formations occur in a definite order in 



i logy of the Black Hills of Dakota, p. 47, 



f Origin of the Ferruginous Schists and Iron Ores of the Lake Superior Region, R. D. Irving : 

 Am. Jour. Sci., 3rd Ser., Vol. X XXII, 1886, pp. 255-272; The Penokee Iron-Bearing Series of Michi- 

 gan and Wisconsin, R. D. Irving and C. R. Van Hise: Tenth Annual Report U. S. Geol. Survey (in 

 press). 



