136 
SILURIC FORMATIONS IN MISSOURI 
the green shales an even greater thickness. Further to the north 
blue shales immediately underlie the black shales. 
From the facts stated it is quite clear that the Siluric strata 
are seemingly continuous above with the Devonic formations 
of the region; but that the black Grassy shales rest in marked 
unconformity upon the bevelled edges of all older rocks, in places 
lying directly upon the Bowling Green limestone. 
The nature of lower medial unconformity is interesting. As if 
it were not enough to have the Siluric section of northeast Mis¬ 
souri delimited above and below by notable planes of uncon¬ 
formity there appear to be two even more remarkable stratigraphic 
breaks running directly through the middle of the sequence. 
When in an earlier reference to the Siluric rocks of the region 
correlation was made with the Niagaran series of Iowa the basis 
was mainly that of lithologic similarity and stratigraphic succes¬ 
sion, because fossils were not then obtained. The lower uncon¬ 
formity was recognized at the time but not its full significance. 
The real significance of the lower medial unconformity appears 
now to be that of an overlap of formations due to appreciable di- 
astrophic movements in the region at the time of deposition. 
The feature of upper medial unconformity are also instructive. 
The evidences of unconformity between the Sexton limestone and 
the rest of the Siluric beneath are not at once apparent. The 
magnitude of the stratigraphic break is made more appreciable 
through generalization. The Sexton limestone does not appear 
to be represented anywhere in the northern part of Pike county, 
Missouri. It is possible that it really does extend into the northern 
part of the county and that some of its lower layers have been mis¬ 
taken for portions of the Bowling Green member. 
The particular type of this unconformity is one of marked 
discordance in sedimentation that is produced by overlap of for¬ 
mations or of marine transgression. The stratigraphic significance 
of this feature is important. Relationships of the contiguous 
formations are such that for the geological age of the Sexton 
terrane there would be suggested correlation with the Late, or 
at least Mid, Siluric deposition of the north, rather than with 
Early Siluric rocks of the south, as regarded recently by Savage. 
Paleogeographical considerations indicate for the Sexton lime- 
14 Missouri Geol. Surv., Vol. IV, p. 30, 1894. 
