96 
Wisconsin Academy of Sciences, Arts, and Letters. 
to permit subterranean solution and excavation of a cavern at 
least 25 feet wide and 20 feet deep in the limestone previously laid 
down here. Then the cavern was filled with fossilifrous shale and 
limestone. Not a trace of this filling or of its fossils is notable in 
the quarry face 30 feet to either side of the cavern. Beyond that 
£ro5m c/iamneh in basal Gale/ra and top cf F/atteyi/fe 
or Seloit /mestone. Dar///7jtan, JVisco/jsift 
distance the layers of limestone seem so perfectly conformable to 
each other that none would suspect that one of the bedding planes— 
indeed it is one of the least evident-—marks a time when this area 
was land and subject to surficial and subterranean erosion. Cases 
like this should make one pause before declaring in error another 
who thinks he sees a stratigraphic hiatus between two apparently 
conformable layers of rock. 
Nor is the relative conspicuousness of the break any reliable in¬ 
dication of the time represented by the hiatus. The time may be 
immeasurably greater than in the case just described, and yet it 
may require very close scrutiny of the purely physical evidence to 
determine precisely where the break occurs or, indeed, whether any 
at all occurred. Of course, with the aid of the fossil evidence not 
only the actual presence and precise location of the break often is 
quickly established, but the fossils also give the best measure of 
the time represented by it. 
The Bevono-Silurian hreak .—So far as known this contact is 
everywhere unconformable, meaning by that only that the sea was 
