KINETIC MEASUREMENT OF THE ELASTICITY OF ROCKS. 17 



is also given in the table. The actual velocities of longitudinal 

 waves in various rocks may differ from those given ; the table, 

 however, will probably furnish a rough estimate which may be 

 of some use in seismometry. 



Expressing the elastic constant of the rocks, classified accord- 

 ing to the age of formation, by means of " the height from a fixed 

 base line," Fig. I, we find a distinct gradation as we pass from the 

 rocks of the Archaean age to those of the Cainozoic. Some of 

 recent age may, of course, have a greater modulus of elasticity 

 than those belonging to the older periods. As a whole, however, 

 Archfean rocks come in first of all, while Cainozoic rocks come 

 in last. The greatest and the least of the Archaean group are 

 greater than those of the Palaeozoic group respectively, and so on 

 in turn for other periods. In the mean, the greater part of 

 Archœan rocks have a greater modulus of elasticity than the 

 greater part of Pakçozoic rocks and so on. 



The modulus of elasticity for a given rock may vary within 

 wide limits as the density and other physical properties differ 

 for each specimen. In the case of granite, e.g., No. 59i has 

 the greatest value ^=5'93xlO" while No. 71"^ the least value 

 E=l'25xlO^\ c.g.s. unit. For engineers, it would be well to 

 remember that in the case of any rock with unusually great 

 density it does not necessarily follow that the modulus of elasticity 

 is correspondingly great. For example, a piece of granite No. 63i 

 has a density twelve percent greater than that of the other piece 

 of granite No. 9i, while the latter has its modulus of elasticity, 

 on the contrary, forty-eight percent greater than the former. The 

 possession of a greater modulus of elasticity and yet less density 

 is what makes a material the more valuable. 



As a general rule, however, rocks of recent formation have 



