52 
IOWA ACADEMY OF SCIENCES. 
Stress in its ultimate analysis is probablj^ dynamic. If so, 
the maintenance of the field of strain about an atom as it moves 
presents no greater difficulty than the maintenance of the field 
of light about a moving candle, or of the field of sound about a 
moving bell. 
The propagation of such ether strains as occur in light, elec- 
tricity and magnetism is very greatly influenced by the material 
substances present in the strained medium. It is not probable 
that the gravitational strain differs from others in this respect, 
and we may reasonably hope to find some inductive phenomena 
in connection v/ith gravitation. A feasible plan is to surround 
a delicately poised mass by a thick pair of hemispheres (which 
may be hollow for liquids), and note with a refractometer any 
change of position, which, since the attraction of a sphere at a 
point within it is zero, will be due either to induction or to 
irregularities of the sphere. Errors due to irregularities may 
be readily eliminated by rotating the sphere. 
THE LE CLAIRE LIMESTONE. 
BY SAMUEL CALVIN. 
The Le Claire limestone constitutes the second stage of the 
Niagara formation as it is developed in Iowa. The first or 
lower stage has been called the Delaware, from the fact that 
all its varying characteristics are well exhibited in Delaware 
county. The Delaware stage embraces many barren beds and 
presents a very great number of phases, but at certain horizons 
it abounds in characteristic fossils. The typical faunas of this 
lower stage embrace such forms as Pentamerus oblongus Sow- 
erby, Halysites catenulatus Linnaeus, Favosites favosus Goldfuss, 
Strombodes gigas Owen, Strombodes pentagonus Goldfuss, Ptyclio- 
pliyllum expansum Owen, and Dipliypliyllum multicauleUdail. The 
beds of the Delaware stage furthermore contain large quanti- 
ties of chert. 
The Le Claire stage of the Niagara follows the Delaware. 
The exact line of separation between the two stages has not 
been, and probably cannot be, definitely drawn. There are 
