ORIGINAL HURONIAN DISTRICT 5 



of the northern woods, can appreciate the physical and mental stamina 

 required for successful geological work under such circumstances. 



On the north shore of lake Huron, Logan and Murray found rocks 

 which antedate the Cambrian, or, as the lowest formations of the fossil- 

 iferous rocks were called in those days, the Silurian. While these rocks 

 were considerably altered, they were not changed from their original con- 

 dition so much that they could not be readily recognized in the field as 

 quartzites, graywackes, slates, conglomerates, limestone, etcetera, and 

 tlierefore the series was regarded by Logan as non-metamorphic. The 

 sedimentary formations on the north shore of lake Huron were found to 

 be arranged in a definite order, allowing as precise mapping as the sedi- 

 mentary rocks of Wales. Below this sedimentary series Logan and Mur- 

 ray found another system, composed mainly of granites and gneisses, 

 which is similar to the rocks comprising the larger masses of the Lauren- 

 tian moimtains. Logan supposed this lower series to be of the same age 

 as the rocks of those mountains, and he therefore carried the name Lau- 

 rentian to the north shore of lake Huron, and there applied the term to 

 the lowest series, although the Laurentian of this area contains no sedi- 

 mentary rocks like the so-called Upper Laurentian, or Grenville, in the 

 original district. But the newer set of rocks, resting on the older series, he 

 recognized to be different in lithological character from any of the rocks 

 of the Laurentian area. He also saw that in the conglomerates of the 

 upper series are very numerous pebbles and boulders identical with the 

 rocks of the older series, and he therefore inferred that the newer series 

 rested unconformably above the older. Because the newer series had not 

 been greatly changed from their original condition and because of the 

 inferred unconformity, Logan pursued the same method he had followed 

 for the Laurentian district — that is, he applied a local term to the set of 

 rocks which he believed to be younger than the Laurentian. Since they 

 skirt the north shore of lake Huron, he natiirally called them Huronian. 

 This was in 1858. 



In 1883 Irving visited the original Huronian area, and found a contact 

 between the lowest Huronian sediments and the Laurentian granites 

 which showed a great imconformity to exist between the two; also he 

 recognized that the chloritic schists of Logan, which he supposed to be 

 metamorphosed sediments, are basic igneous rocks, largely volcanic. 



Later, Pumpelly and I, studying the Huronian series somewhat closely, 

 found an unconformity within the Huronian sedimentary series between 

 Logan's limestone formation and the upper slate conglomerate. In 1902 

 further close studies by Leith and myself showed this unconformity to be 

 important; also we found the chloritic schists which Logan placed with 



