^YOEK OF LOGAN AND OTHERS / 



and which, incidental!}^ it may be remarked, is about a twentieth part as 

 accurate. 



We have here an illustration of an error which has crept into geology 

 again and again, not only with reference to the pre-Cambrian, but with 

 reference to other formations. As a result of patient, careful work, 

 taking into account all the facts, a stratigraphic problem is worked out. 

 Not one criterion, but many are used. But perhaps one is conspicuous. 

 This is selected and applied broadly in other regions, ignoring all others. 

 A-s a result, the rocks grouped under the method are classified upon a 

 certain basis, but this is no longer the original basis. If the geological 

 maps which have been produced by the color method of mapping had been 

 so called, so that it was understood that the term Laurentian meant acid 

 granitic and gneissic rocks, and that Huronian meant quartzites, gray- 

 wackes, conglomerates, and basic igneous rocks, without reference to rela- 

 tions, no harm would have been done. The defect consisted in regarding 

 this work as structural. 



The Lake Supeeige Eegion 

 review of the investigations in this field 



After having worked oi;t the stratigraphy of the north shore of lake 

 Huron, Logan visited lake Superior, and there recognized a group of 

 rocks in which the copper deposits occur, which he regarded as different 

 from and at a higher horizon than the Huronian. He called these rocks 

 the upper copper-bearing series in contrast to the Huronian of the north 

 sliore of lake Huron containing the copper deposits of Bruce mines. This 

 upper series is the one which was later known as the Keweenawan. 



We must now pass from Canada to the United States to find the con- 

 tinuation of the early methods of work of Logan and Murray. In Mich- 

 igan and Wisconsin, state geological surveys were organized. In Michigan 

 the work on the copper-bearing rocks was handled by Pumpelly and Mar- 

 vine, and that upon the iron-bearing rocks by Brooks. Brooks, Irving, 

 and Wright were all engaged in studying on the iron-bearing rocks of 

 Wisconsin. In these districts there were great economic deposits of iron 

 and copper which justified the most careful mapping. 



Prior to the investigations by the state surveys in this region, a large 

 amoimt of good preliminary work had been done by Poster and Whitney 

 and others, but this work was of the same general type as that of other 

 American geologists antecedent to the days of Logan and Murray. These 

 geologists and their defender, Wadsworth, insisted that the pre-Cambrian 

 rocks were indivisible on a structural basis and to all of them the term 



