52 IOWA ACADEMY OF SCIENCES. 



Stress in its ultimate atialysis is probably 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 thematerial 

 substaLces 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 with 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 chtiracteristic fossils. The typical fauDas of this 

 lower stage embrace such forms as Pentamerus oUongus Sow- 

 erby, Haly sites catenulatus Linn sens, Favosites favosus Goldfuss, 

 Strombodes gigas Owen, Strombodes j^entagonus Goldiass, Ptycho- 

 phyllum expansum Owen, and Diphvi)hyllum multicauleB.^ll. 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 



