332 21ie Autertcan Geohjyia'. Doccmhor, L><95 
partisMii and souietiuK-s hostile schools, did not I'acilitate the 
solution of the structural problems that confronted the geolo- 
gist at this hoVizon fifty years ago, but they all conspired to 
throw about the geology of the sub-Silurian for many years an 
impenetrable fog which no one was willing to attempt to 
pierce. Later these problems have arisen in the Lake Supe- 
rior region, under different conditions, and they have been 
gradually unfolding under the labor of numerous workers, 
spread over a much wider area and advancing with greater 
precision and deliberation, with more means at their disposal 
and better geological training to fit them for the task. If they 
have made more satisfactory headway, with more co()peration 
and harmony, it is because of dilferent stimulants and more 
genial conditions rather than more skill or greater industry. 
Furthermore, they have had the experience of their predeces- 
sors, both as a guide and as a warning. 
The following table exhibits at a glance the contrasts that 
appear between the classification of Messrs. Walcott and Van 
Hise and that of the writer. The geologist who has followed 
the argument of tliese papers will have learned what are the 
main reasons for not accepting the taxonomy of the "Correla- 
tion i)apers''' lately issued. If these objections are valid, there 
is a necessity for revision and careful re-examination in the 
field at points where the crucial facts can be seen. 
There are two leading and fundamental differences between 
these classifications, which tlo not relate in the least to the 
question of nomenclature, although nomenclature may have 
l^een one of the original elements predisposing one way or the 
-other. These are : 
1, The existence or not of a great erosion interval between 
two sandstones, viz., between the upper member of the Kewee- 
nawan, which consists of red erosible sandstones, and the bot- 
tom of the horizontfd sandstone, which, excepting its basal 
conglomerate at overlap contacts, also consists of red erosible 
.shales and sandstones. 
2. The integrity or the dismemberment of the Keweenawan. 
In reference to the first, while it is not necessary to rehearse 
the argument in full, it may be well to repeat that all the 
non-conformable contacts of the horizontal sandstone on the 
trap of the Keweenawan, and also those on the quartzytes at 
