JReviews — Professor W. S. Hobbs on Earthquakes. 131 



, Conditions of space render it impossible to notice more than 

 a selection of the new and interesting forms treated. We trust, 

 however, that enough has been written to show what a wide field has 

 been ably traversed by Dr. Perner, and to awaken the desire to study 

 his elaborate work in extenso. 



Jane Longstapp (Donald). 



y. — Earthquakes : An Introdtjction to Seismic GEOLoar. By 

 William Heebeet Hobbs, Professor of Geology, University of 

 Michigan. New York, 1907. 



rilHE work before us is one deserving of special attention from 

 X geologists. It cannot be denied that recent and very important 

 advances made in the study of earthquakes have tended to throw 

 the science of Seismology into the domain of the physicist and 

 mathematician, rather than into that of the naturalist. The con- 

 struction of delicate recording instruments, the unravelling of the 

 complicated results of different kinds of wave- movement, and the 

 discussion of the conclusions to be drawn from these as to the nature 

 and disposition of the materials entering into the composition of our 

 globe, make seismology (like the study of underground temperatures/ 

 terrestrial gravity, or terrestrial magnetism) an important branch of 

 geo-physics. But while this aspect of the subject is not lost sight 

 of, it should ever be remembered that the geologist has at least an 

 equal claim with the physicist to be heard in the discussion of 

 seismological problems. The records of the history of the earth's 

 crust, as studied by the geologist, supply evidence concerning the 

 nature and the effects of seismic action which cannot be neglected 

 if we are to obtain the fullest possible amount of light upon the 

 subject. It is true that the idea, formerly held by geologists, that 

 there is a direct connexion between volcanic and earthquake 

 phenomena, has been steadily losing ground ; but, on the other 

 hand, the intimate relations between earthquake movements and the 

 geological phenomena of jointing and faulting — or, as our author 

 prefers to express it, the effects of constant readjustment of blocks 

 of the earth's crust to one another — are coming to be regarded as 

 the essential factor in all great earthquakes. The interpretation of 

 the phenomena of the past, by the study of forces in action at the 

 present time, which is so fully recognised as the true principle of 

 reasoning concerning the external agents operating on the earth's 

 crust, is now felt to be equally applicable to the internal forces 

 acting upon it. ' Seismic geology,' though divorced from Yulcanology, 

 must still be regarded as one of the essential branches of Geological 

 Dynamics. 



In the earlier chapters of this work the author, inverting the 

 usual method of treating the subject, deals with the evidences so 

 familiar to the geologist of constant strain and intermittent fracture 

 within the earth's crust, and demonstrates the existence of unstable 

 belts where such conditions attain their maximum. He then shows 



