16 J'he American Geologist. July, 1895 
er arc, with some hesitanc}', made the equivalent of the Pots- 
dam or of the Potsdam and Calciferous of New York state. 
But a serious error w^as committed by Logan in this compi- 
lation, as has been pointed out distinctly by R. D. Irving. It 
consisted in not parallelizing the rocks of the original Hiiron- 
ian with the lower volcanic group (Animikie) of the copper- 
bearing series, in the vicinity of Thunder bay, and making 
them the equivalent of the lower slates at that point which 
are non-conformable below the lower volcanic group. This 
separation of the lower volcanic group in the region of Thun- 
der bay from the Huronian necessitated the application of 
the term Huronian, by him and b}- all subsequent Canadian 
observers up to the time of A. C. Lawson, to the underlying 
schists, which has finally led to the extension of the term to 
all the schists and associated rocks downward to the Lauren- 
tian, and, further, to the assertion that the Huronian is con- 
formable with the Laurentian, since these schists gradually 
merge into the gneisses which constitute the Laurentian in the 
Lake Superior region. Again, since these lower schists, with 
their associated greenstones, are by far the greater series and 
the most important of the schistose rocks of the region, they 
came soon to monopolize the term Huronian, and to determine 
its significance, thus greatly perverting its original sense and 
scope. It was this perversion that Irving and Lawson cor- 
rected.* The separation of these lower schistose and gneis- 
sic rocks from the term Huronian, and their designation un- 
der the new term Ontarian by Lawson, is a recent important 
classificatory step. However, these corrections have taken 
place since the Wisconsin survey. 
With the exception of the non-identification of the Huron- 
ian with the copper-bearing rocks (both upper and lower 
groups), the later grouping by Logan agrees with his earlier. 
The effect, however, of his earlier publication remained with 
the British geologists, and remains to this day. They recog- 
nized and adopted Logan's term, Laurentian, but they make 
the Cambrian the equivalent of his Huronian. 
Some 3'ears before Logan's classifications, as above, some 
United States geologists were surveying the copper and iron 
♦Compare N. H. AYiNCHEi.L, Methods of stratigraphy in studying the 
Huronian. Amkhican Geologist, vol. iv, pp. 342-357, 1889. 
