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HISTORICAL SKETCH—LAKE SUPERIOR REGION.
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carbonaceous and ferruginous shales and slates. The upper slate formation includes the great mass of the Upper Huronian series. Its maximum thickness is more than ten thousand feet.

In certain areas, during Upper Huronian time, there was great volcanic activity, as a result of which, peculiar formations were piled up, wholly different from any of the ordinary members of the series. Also this volcanic activity greatly disturbed the regular succession, so that for each of the volcanic districts an independent succession exists, the sedimentary and volcanic formations being intimately interlaminated. The two areas which are best known are the Michigamme iron district north of Crystal Falls and the east end of the Penokee district. Similar volcanics also occur in the Marquette district. In the Michigamme iron district is an extensive area of greenstones, greenstone-conglomerates, agglomerates and surface lava flows, many of which are amygdaloidal. In the Penokee district the materials are almost identical. The typical succession for this district extends in unbroken order for fifty miles or more, but east of Sunday Lake this is suddenly disturbed by the appearance of the volcanics. The character of the rocks and their order soon becomes so different that if one were not able to trace the change from one into the other, there would be a great temptation to regard the part of the series bearing volcanics earlier than or later than the Penokee series proper. But the continuity of the two cannot be doubted. Thus this occurrence well illustrates that lithological character in pre-Cambrian, as in post-Cambrian time is no certain guide as to relative age. Finally, associated with the Lake Superior Upper Huronian rocks are many later intrusive dikes and interbedded sills, chiefly diabases, gabbros and diorites, but local granitic intrusives also occur, particularly in the Felch Mountain and Crystal Falls districts, and possibly also in the Menominee district.

The typical districts in which the Upper Huronian series can be best studied are the Penokee, Marquette, Mesabi and Animikie. Remote from the Lake Superior region proper, the rock series

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