Gabbroid Rocks of Mimiesota. — [ I 'incliell. 1 5 9 
of the state (and of the world) are the greenstones, which 
are both massive, igneous, and clastic. With them are asso- 
ciated schists, gneisses, granites, quartz porphyries, and jas- 
pilytes (iron ore rock) in the Lower Keewatin. The gneisses 
and granites placed in the Laurentian by the United States 
Geological Survey are considered by the Minnesota Survey to 
be of later date than the greenstones, and to be derived from 
elastics by metamorphism. An unconformity followed b}' the 
Ogishkie conglomerate separates the Lower from the Upper 
Keewatin, which consists of a series of argillytes, graywackes, 
etc. These two divisions of the Keewatin together constitute 
the Archean in Minnesota, and are followed by a great un- 
conformity. The Keewatin formations are invariably nearly 
vertical in position; the succeeding series dips at a com- 
paratively low angle, or may even be horizontal. The oldest 
terrane of the Paleozoic is called theAnimikie in Minnesota ; it 
is composed of quartzytes, slates, etc., and contains the richest 
iron mines in the world. The Keweenawan, in its widest sense, 
is composed of two members : the Cabotian^ and the Manitou. 
The lower division (Cabotian) rests upon the Animikie, per- 
haps unconformably. It is composed in Minnesota exclusively 
of igneous rocks, and it is a name intended only for such 
rocks ; but it represents a time interval elsewhere occupied by 
sedimentaries. The Keweenawan^ as a time interval, should 
be limited to the epoch immediately following the Puckwunge 
conglomerate, which marks the unconformity existing be- 
tween the Cabotian and the Manitou. The latter is a term 
also intended to designate igneous rocks, but even in Minne- 
sota sedimentaries begin to appear from the first dawn of the 
epoch, and towards its close sandstones predominate. These 
three: the Animikie, Cabotian and Manitou, together con- 
stitute the Taconic, which is equivalent to the Middle and 
Lower Cambrian. The upper sandstones of the Manitou pass 
insensibly into the sandstones of the Upper Cambrian, whose 
later members show characteristic fossils. 
If this classification be correlated with that adopted by the 
United States Geological Survey, fundamental differences will 
be found, especially in regard to the series of gabbros which 
are to be studied in this article. It must be noted in the first 
place that correlations are always difficult and dangerous, and 
