SILURIC FORMATIONS IN MISSOURI 
137 
stone Late Siluric age, as is well shown by some of the more 
recent paleogeographic maps of North American continent by 
Schuchert^® and Willis.^® 
In considering the correlation of the several formations the 
genetic affinities of lower part of section is most important. That 
portion of the Siluric section beneath the lower medial plane of 
unconformity — the oolite and its equivalent, the yellow limestone, 
both highly fossiliferous — appears to become rapidly thicker 
towards the south. New beds come in below. In southern Mis¬ 
souri a great series is formed the main member of which is a ter- 
rane widely known as the Girardeau limestone. 
According to Savage this southern sequence constitutes the 
main part of the Alexandrian series and is regarded as Early 
Siluric in age. By slight emendation of the original proposal 
so as exclude all the Siluric above the lower medial unconformity 
the term Alexandrian Series becomes a fitting and valid title for 
the rocks under consideration. 
The stratigraphic equivalency of middle part of section is no 
longer uncertain. Above the lower medial plane of unconformity 
the Siluric sequence is heavy, buff, or brown dolomite. It carries 
only a few fossils. The formation becomes thicker to the north 
of Pike county. Through means of deep-well records it is easily 
traceable into northern Iowa. This part of the section is therefore 
paralleled with Mid Siluric succession of that state.^® There the 
Niagaran series comprises four distinct dolomites — the Sabula, 
Colesburg, Hartwick, and Monticello formations.^® 
The Bowling Green dolomite apparently has no recognizable 
equivalent in south Missouri. The organic remains which are 
found in it are few in number as well as species, and are such 
forms which have extended vertical range. For this reason the 
fossils are by no means decisive factors in the detailed parallelism 
of the beds. Other correlative criteria have to be mainly relied 
upon. 
The correlation of upper part of section brings in another 
problem. The Siluric section above the upper medial uncon¬ 
formity seems illy correlated with the succession of the Early part 
15 Bull. Geol. Soc. America, Vol. XX, p. 533, 1910. 
16 U. S. Geol. Surv., Prof. Pap. 71, p. 226, 1912. 
17 Am. Jour. Sci., (4), Vol. XXV, p. 434, 1908. 
IS Am. Jour. Sci., (4), Vol. XXXVIII, p. 256, 1914. 
19 Proc. Iowa Acad. Sci., Vol. XIX, p. 149, 1913. 
