IOWA ACADEMY OF SCIENCES. 
177 
says of the Nathrop crystals; “They are evidently not second- 
ary like zeolites, but primary and produced by sublimation or 
crystallization from presumably heated solutions contempora- 
neous or nearly so with the final consolidation of the rock.” It 
is very likely that the Utah topaz was formed in the same way. 
FORMATION OF THE FLINT BEDS OF THE BURLING- 
TON LIMESTONES. 
BY FRANCIS M. FULTZ. 
[Abstract.] 
For convenience the flint beds of the Burlington limestones 
are usually separated into two divisions, known as the “lower” 
and “upper” flint beds. The lower series is probably from 
fifteen to twenty feet thick and the upper a little more. They 
have always been classed as the latest deposits of the Lower 
Burlington and Upper Burlington limestones, respectively. 
The lower beds are not so continuously chert as the Upper; in 
fact much of the deposit is siliceous shale mixed with clay and 
containing thin bands of limestone. There are also certain 
strata which look like heavy-bedded limestone, but which con- 
tain so much siliceous and argillaceous matter as to be utterly 
worthless. As already stated the upper series is somewhat the 
thicker. There is also a much greater proportion of chert and 
much less siliceous shale, while the amount of limestone remains 
about the same. 
As yet no very satisfactory theory as to the origin of these 
flint beds has been advanced. Certainly none satisfactory 
enough to be generally accepted. The investigation to which 
attention is here called has been mainly as to whether the 
origin of the material has been chemical or organic. So far the 
preponderance of evidence is in favor of the former. 
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