INTRODUCTION
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![](http://upload.wikimedia.org/wikipedia/commons/thumb/3/3a/NASA_SP-56_Microfossils_from_Precambrian_chert_2.jpg/360px-NASA_SP-56_Microfossils_from_Precambrian_chert_2.jpg)
Figure 1.—Representative microfossils, three-dimensionally preserved in chert from the Gunflint iron-formation of the north shore of Lake Superior. This formation is of middle Precambrian age and has been dated by K40-Ar40 as approximately 2 × 106 years. All figures are from thin sections of the chart photographed in transmitted light. Published here for the first time (from Dr. E. S. Barghoorn).