Mr. Matter on the Dynamics of Earthquakes. 105 
Fig. 8 shows, by four diagrams, the nature of the forces of extension and compression, 
and the directions of the elastic waves produced either by flexure or fracture. 
In f, the mass of stratified formation is in act of being elevated by the protrusion of 
lava from beneath, carrying up the loose materials reposing upon it. The arrows above 
and below the neutral plane indicate the directions of the compressions and extensions, and 
hence of the transit of elastic waves, if the flexure be of sufficient extent and rapidity to 
produce such. In y, a similar stratified mass is supposed to be in progress of subsidation, 
and the arrows show that all the previous forces remain operative, but changed in direc- 
tion. In 8, the flexure has been attended with fracture, and earth waves have passed 
outwards in all directions, and the diagram shows the directions of compression produced 
by the wedging together of masses of rock in progress of being lifted or tilted over, by 
continued upheaval after previous fracture, by which minor earth waves, subsequent to the 
great shock, may be propagated. 
Fig. 9 has been explained in referring to the map of cotidal lines, Plate I. 
VOL. XXI. 1d 
