104 Adams and Coker — Elastic Constants of Rocks. 



schistose, porous or loosely coherent, the method will, from the 

 nature of the case, be very much less satisfactory. 



The plutoiric igneous rocks as a class most nearly resemble 

 the metals in structure, being holocrystalline and massive, and 

 therefore present the nearest approach among rocks to perfectly 

 elastic bodies : they are therefore a class of rocks to which this 

 method is especially applicable. It fortunately happens that 

 they also form a class of rocks a knowledge of whose com- 

 pressibility is of special importance for the elucidation of 

 geological problems, constituting as they do the greater part of 

 the earth's crust. 



A second class of rocks which are comparable to them in 

 their approach to perfect elasticity, comprises the marbles and 

 certain limestones. 



A series of sixteen typical rocks representative of these two 

 classes were accordingly selected for measurement ; under the 

 first class a number of granites were chosen as representing 

 the acid plutonic rocks, and a number of types of the gabbro- 

 essexite series were selected as representing the basic plutonic 

 rocks. In all cases great care was taken to choose the most 

 homogeneous and massive rocks of each series, and to secure 

 test pieces free from all flaws and cracks. As representing 

 the second class a number of typical marbles and limestones, 

 also perfectly massive in character, were selected. For pur- 

 poses of comparison, or contrast, a sandstone was added to the 

 list as being a rock which, on account of its more or less porous 

 nature, could hardly be expected to yield satisfactory results 

 by this method. 



An examination of the stress strain curves of these 16 rocks, 

 omitting the sandstone, shows that on the average they possess 

 a rather more perfect elasticity and exhibit less hysteresis than 

 cast iron. Some of them, as for instance, the Baveno granite, 

 the nepheline syenite, the diabase and the black Belgian 

 marble, show much better curves, approximating in fact to the 

 straight lines given by wrought iron, which may be considered 

 for our present purpose as expressing perfect elasticity. 



The close approximation to perfect elasticity is shown by 

 the return of the curve to its initial or starting point, and the 

 amount of the hysteresis is shown by the width of the loop. 



The width of this hysteresis (or lag) curve or loop indicates 

 the amount of the divergence from Tlooke's law which the 

 material exhibits — this law being that the stress and strain are 

 directly proportional. When the curve is narrow, as it is in 

 all cases except the Stanstead granite and the sandstone, the 

 divergence from Hooke's law is not great enough to seriously 

 affect the result. 



The rocks, therefore, with these exceptions, fulfil the condi- 

 tions of elasticity necessary to the successful application of the 



