Geology and Mineralogy. 331 



6. Maryland Geological Survey, Vol. V, 1905. Wm. Bullock 

 Clark, State Geologist, pp. 656, pis. 35, figs. 55. — The present 

 volume is largely devoted to economic subjects and each of the 

 parts have been issued separately at different times. 



The Second Report on Magnetic work in Maryland by L. A. 

 Bauer, now in charge of the magnetic work of the IT. S. Coast 

 and Geodetic Survey, forms Part I of the volume. This mag- 

 netic survey of Maryland is the most complete of any magnetic 

 survey in the world except that of Holland, the magnetic declin- 

 ation, inclination and force having been determined in every 

 portion of the State. The results have proved of both great 

 practical value and scientific interest. The plate showing the 

 lines of equal magnetic declination indicates their extreme irregu- 

 larity, a pronounced focus of magnetic disturbance existing 19 

 miles northwest of Washington, a local variation of 7 degrees 

 occurring within 7 miles. It is stated that a mathematical 

 analysis of the forces producing the disturbances in this locality 

 traces their source to parallel ridges running approximately in a 

 northeast and southwest direction, agreeing with that of the ser- 

 pentine beds, as mapped by the geologists. 



The report also contains among other papers a report on the 

 highways of Maryland and more than 400 pages on the distribu- 

 tion, value and geology of the Maryland coals. 



This volume, like the preceding ones of the series, is hand- 

 somely printed, illustrated and bound. J. b. 



7. Les Tremblements de Terre. Geographie Seismologique, 

 par F. de Montessits de Ballore. Preface par A. de Lapparent, 

 pp. 500, pis 3, 89 page maps and figures; Paris, 1905 (Librairie 

 Armand Colin). — The author of this volume has been known for 

 many years as one who has patiently and laboriously collected 

 all available statistics regarding earthquakes over the whole 

 world, and he is one of the small group of men to whom the new 

 science of seismology owes much. In this publication the whole 

 subject of the geographic distribution of earthquakes is clearly 

 and fully discussed.. The numerous detailed maps occurring at 

 intervals through the volume show graphically the location and 

 frequency of recorded shocks. The method of representation 

 has been to cover each town or center of seismic disturbance by- 

 a black circle, the length of the radius of the circle being propor- 

 tional to the number of shocks. This method has been substi- 

 tuted in place of graded tints, bounded by curves of seismic fre- 

 quency, the departure having been made on account of the essen- 

 tially discontinuous and localized nature of earthquake centra. 

 This has its advantages for the student of seismology, giv- 

 ing greater precision to the graphic representation of the data, 

 but also possesses its disadvantages, especially for the casual 

 reader. The area of a black disc impresses the eye rather than 

 the radius, giving the impression of a greater concentration of 

 shocks at certain centers than is actually the case. Again, as the 

 time of observation grows longer the number of scattered shocks 



