118 



IGNEOUS AGENCIES. 



several hundred feet in the air, and afterward were found across a river 

 and on the top of a hill. In earthquakes of this kind — 1. The 

 impulse is very powerful and sudden, so as to make a high but not 

 a long wave, or, in other words, the velocity of vibration or of the 

 shock is very great; and, 2. The focus is not deep, so that the 

 velocity of the shock-motion does not become small before it reaches 

 the surface. At Riobamba the velocity of the shock-motion was still 

 very great when the wave reached the surface. From the distance 

 bodies were thrown, Mallet supposes the velocity of the shock-motion 

 could not have been less than eighty feet per second (Jukes). 



The horizontally progressive hind may be regarded as the true 

 type of an earthquake ; it is in fact the spreading surface- wave al- 

 ready explained. If the elasticity of the earth, and therefore the 

 velocity of the waves, is the same in all directions, the surface-wave 

 will spread in concentric circles; but if the elasticity, and therefore 

 the velocity of the waves, be greater in one direction than in another, 

 as, for example, north and south than east and west, or the converse, 

 then the form of the outcrop will be elliptical. In some rare cases the 

 shock seems to run along a line. Thus progressive earthquakes have 

 been subdivided into circular, elliptical, and linear progressive. We 

 have already given the simple explanation of the first two; the last 

 may be briefly explained as follows : 



Let it be borne in mind : 1. That these linear earthquakes usually 

 run along mountain- chains; 2. That most great mountain-chains consist 

 of a granite axis (appearing along the crest and evidently connected be- 

 neath with the great interior rocky mass of the earth), flanked on each 

 side with stratified rocks consisting of many different kinds ; 3. When 

 elastic waves pass from one medium to another of different elasticity, 

 in all cases a part of the wave passes through, but a part is always 

 reflected. For every such change — for every layer — a reflection occurs ; 

 and, therefore, if there are many such layers, the waves are quickly 

 quenched. If, now, Fig. 100 represent a transverse section across 



Fig. 100.— Diagram illustrating Linear Earthquakes. 



such a mountain, and X the focus of an earthquake, it is evident that 

 the portion of the enlarging spherical wave which emerged along the 



