i6o The American Geologist. September, 1900 
that they can only represent in a very general way the various, 
and often complicated equivalencies. Within these limitations, 
however, an approximate correlation seems possible, and is ex- 
pressed in the following tabulated form . 
The feature of the classification of the United States Geo- 
logical Survey which is most distinctive, fundamental, and 
conspicuous is the introduction of the Algonkian (Precam- 
brian) system, which is not recognized by the Minnesota sur- 
vey. The Archean in Minnesota is thus limited to a portion 
of the Lower Keewatin, composed of the rocks that are so old 
as to show no clastic characters, that is, the main body of the 
greenstones, with the oldest schists and gneisses. The Archean 
is divided into two groups wholly on lithological grounds. 
The Laurentian includes the granites and granitoid gneisses ; 
the Mareniscan embraces the fine grained mica schists, horn- 
blende schists, hornblende gneisses, and even some felds- 
pathic schists. The distinction is clearly somewhat arbitrary, 
and the two divisions frequently pass one into the other by 
imperceptible transitions. But in Minnesota they are said to 
be separated by an eruptive unconformity. The Lower Hu- 
ronian rests unconformably upon the Archean, and is separat- 
ed by another unconformity from the upper Huronian. The 
first unconformity is not recognized by the Minnesota survey ; 
the second separates the Lower from the Upper Keewatin. 
The Lower Huronian comprises the jaspilytes, quartz porphy- 
ries, and the clastic portions of the greenstones, which latter 
are considered to be quite rare and unimportant. The Upper 
Huronian includes the argillytes, certain quartzytes, gray- 
wackes, etc., and its basal member is the Ogishkie conglom- 
erate/ The great unconformity between the Upper Keewatin 
and the Animikie is not recognized by the United States Geo- 
logical Survey in the Upper Huronian ; or, rather, perhaps, it 
is placed at a higher level between the Huronian and Kee- 
weenawan. The Lower Keeweenawan, though, necessarily 
tabulated as the equivalent of the Cabotian, is not so in fact, 
since it includes various sandstones and conglomerates not 
admitted in the latter. The unconformity above the Cabotian 
is not recognized in the Keeweenawan by the United States 
survey, though it is stated that an unconformity probably ex- 
ists. But it is at the upper limit of the Keeweenawan that a 
