84 The American Geolofji.^f. February, 1895 
only Devonian roek (and this is doubtfully assiji;ned to that 
age) is the Eureka shale, which is a stratum ol" black shale, 
varying fron 10 to GO feet in thickness. It is found only in 
the extreme soutliwestern corner and there lies between the 
magnesian limestones of the Ozark stage and the Lower Car- 
boniferous rocks. 
Lower C'arbonifcrous limestones and shales in great thick- 
ness are exposed over wide areas in the northeastern, central 
and western sections. They are generally in direct contact 
with beds of the Ozark stage. Sometimes, Devonian and 
higher Silurian strata intervene. 
The Coal Measures cover the whole northwestern, and a 
broad strip in the southwestern portions of the state. They 
rest unconformably upon the Lower Carboniferous and extend 
beyond the limits of the latter formation. 
From this brief description it will be seen tiiat the rocks of 
the different formations surround the Archean nucleus in a 
somewhat concentric form, though the sequence is broken in 
many places. This center is geologically a quaquaversal arch 
which has been raised and depressed several times. It is well 
known as the Ozark uplift. 
With these explanatory remarks we will now proceed to de- 
scribe the conditions which prevailed and the noteworthy 
events which took place during the different geologic eras or 
periods, beginning with the oldest. 
The Archean Era. — The Archean land surface of this por- 
tion of the globe must have been a very extensive one. At the 
beginning, at least, it probably spread well beyond the state 
limits. Its original outlines are at present undeflnable, but. 
from the fact that the rocks of the present land must origi- 
nally have been derived in large part from these pre-existing 
Archean rocks, the mass exposed to denudation must have 
been very great. 
The A/gonkfari Era. — Before the end of the Algonkian era 
the Archean land surface of Missouri was entirely submerged. 
Whether this condition was readied during the late Algon- 
kian or during the early Algonkian we are unable to say. 
Probably there was a gradual lowering, such that complete 
submergence was not accomplished till towards the end. The 
extent of the Algonkian deposition is unknown and iindeter- 
