IOWA ACADEMY OF SCIENCE 
173 
A STUDY OP THE CHERTS OP THE OSAGE SERIES OP THE 
MISSISSIPPIAN SYSTEM. 
BY FRANCIS M. VAN TUYL. 
The presence of a considerable amount of chert in the Burlington and Keokuk 
limestone members of the Osage series has been the cause of frequent comment, 
but no critical study of the material has ever been made. 
The chert attains its maximum development in the upper division of the 
Burlington limestone known as the Montrose chert. But nodules and inconstant 
seams of the same material appear, also, in the lower levels of the . Burlington 
and frequently characterize the Keokuk limestone. In the latter formation, 
however, the chert is found at no definite level and is entirely wanting at some 
localities. 
The present investigation was confined mainly to a study of the Montrose 
chert, although some attention was given to the chert from other horizons. 
Outcrops of the Montrose chert may be studied satisfactorily in the neighbor- 
hood of Augusta, Iowa. But excellent facilities for studying the unweathered 
chert were afforded, also, in the power plant excavation at Keokuk. Dis- 
continuous bands and lentils of chert ranging up to 18 inches, or more, in 
thickness occur at this level. 
Crinoidal limestone is associated With the chert of this age and at times this 
may predominate, but usually the chert is in excess. In some layers chert ia 
the dominant constituent and it often incloses pockets and rounded masses of 
limestone. On the other hand, layers of limestone inclosing nodules of chert 
frequently appear. Between these two types uf occurrence there is every 
graduation. 
The crinoidal texture of the limestone, and even its color, are sometimes 
faintly exhibited by the chert, and occasionally fragments of the stems of 
crinoids and other fossils, which may or may not be silicified, are imbedded in 
the material. Some of the chert, however, show^s no trace of fossils although 
it may occur in a very fossilife.rous limestone. Megascopically, the contact of 
the chert with the limestone seems to be fairly abrupt, but chemical and 
microscopic studies of the material show that the silica permeates the limestone 
some distance back from the contact. Upon weathering, the chert cleaves from 
the limestone on account of differential expansion and contraction, and if 
calcareous fossils are present these are soon dissolved out to leave moulds. 
Exposure, also, tends to render the chert brittle and rotten and large masses 
sometimes break down readily into heaps of small angular blocks. 
Microscopic study of the material shows it to be for the most part, of the 
chalcedonic variety of quartz, but some of the silica is apparently in an opaline 
condition.. 
With respect to the origin of the cherts of the Osage series there have been 
differences of opinion. HoveyS who made a study of the cherts of Missouri for 
the Missouri Geological Survey, examined some material from this horizon. 
