62 THE PENOKEE IRON-BEARING SERIES. 



(3) As to the Huroniau : 



^ynopiical notes on Huronian formation. — Name derived from lake Huron, on the 

 north .side of wliich the formation is well developed. Known in Wisconsin and ]Mieh- 

 igan as the Iron l>earini;- formation. Probal)ly embraees also the great iron deposits 

 of Missouri, New York, and Canada. Consists of a variety of metamorphosed sedi- 

 ments, embracing quartzites, limestones, clay slates, micaceous, hornblendic, carbona- 

 ceous, and magnetic schists, and diorites and ]wrphyries of doubtful origin. Thick- 

 ness 13,000 feet, more or less. Strata arched and sometimes folded, but not usually 

 closely crumpled and compacted like the Laurentian. Constitutes the Penokee, 

 Menominee, and Black river iron ranges, the quartzite and porphyry outliers of central 

 Wisconsin, and the quartzites of Barron and Chippewa, and probably of Marathon 

 and Oconto counties. Existence of life probable. . . . 



Huronian (jcofjraphij. — At length the unrevealed interval gave place to a known 

 era. In the jirogress of erosion and subsidence the sea advanced upon the Lauren- 

 tian lands, and sei)arated from them a large island within our northern l)oundaries, 

 and two or three smaller ones, as it would seem, in the adjacent territory of Michi- 

 gan. ... 



Local characteristics — Fenolcec region. — Along the Penokee range the Huronian 

 beds are found abutting against a wall of Laurentian rock, whu'h formed the ancient 

 shore line, and definitely marked the southern limit of the primitive Superior seaJ 

 Here we lind a series of Huronian beds nearly 13,00t» feet in thickness. These are 

 now upturned and metamorphosed, but the history of their formation remains for the 

 most part legible. 



The Penokee series — 1. Limestone. — The lowest member exposed to view is a 

 crystalline maguesiau limestone 130 feet in thickness; the earliest limestone known 

 in our series. Its bedding and its association with aqueous sediment show that it 

 was deposited beneath water as a calcareous sediment. The source of its material 

 deserves special consideration. The student will perceive, on a moment's reflection, 

 that neither the simple decay nor the wear of the adjacent Laurentian rocks would 

 give a material made up almost wholly of lime and magnesia, for the Laurentian 

 rocks contain these ingredients only in very subordinate quantity, and, furthermore, 

 these are among the ingredients removed — not left — by decay. The ordinary seili- 

 ments resulting from decay and wear are clays and sands, not limestone. 



In later ages there is the clearest evidence that the great limestone formations 

 were made from the calcareous remains of marine life in ways that will appear more 

 clearly as we proc'ced. It is probable that the ancient bed of limestone under con- 

 sideration was formed in a similar way, although no distinct traces of fossils have 



' This was not then a vertical wall as it now appears, hecause it has since been disturbed in common 

 with the Huronian strata. But if the latter be depressed *o their original po.sition, the Laurentian slope 

 where observed would bo about 30'^, which may be taken as the declivity of the Laureutian shore. 



