Reviews 



The General Magnetic Survey of the Earth. By L. A. Bauer. Bull. 

 Am. Geog. Soc, XL VI, July, 1914, pp. 481-99. Figs. 6. 

 About the earth sphere are lines of magnetic force very similar to 

 those of any magnetic field, their poles not quite coincident with those 

 of the earth axis. That the magnetic needle varies from true north 

 was discovered at least as early as the fifteenth century, when Columbus 

 sailed west from Europe. Subsequent observations have shown in 

 addition that there is a constant change in the earth's magnetism, mak- 

 ing repeated magnetic surveys necessary. In magnetic observations, 

 the horizontal declination, vertical magnetic dip, and intensity of the 

 attraction are measured. Since 1904 the Carnegie Institution of Wash- 

 ington has conducted extensive magnetic surveys of the earth in which a 

 total mileage of approximately one million miles has been traveled. 

 Ocean surveys have been conducted in a specially constructed non- 

 magnetic vessel. 



R. C. M. 



The Mud Lumps at the Mouths of the Mississippi. By Eugene W. 



Shaw. U.S. Geol. Surv., Prof. Paper 85, Part B, 1913. Pp. 



17, pis. 3, figs. 6. 

 The territory within a mile or two of each of the mouths of the 

 Mississippi is characterized by large swellings or upheavals of tough 

 bluish-gray clay, to which the name "mud lumps" has been applied. 

 Many of the mud lumps rise just off-shore and form islands having a 

 surface extent of an acre or more and a height of 5 to 10 feet, but some 

 do not reach the water surface. Almost all occur near the bars at the 

 mouths of the rivers. In contrast with the general structure of the delta, 

 which is composed of thin, alternating sandy and clayey beds, the mud 

 lumps are of thick, compact clay. On and around the clay core lies a 

 series of faulted and folded strata of sand and silt which have been 

 carried up from the sea bottom and deformed in the upheaval. It seems 

 most likely that these lumps owe their origin to a squeezing of the soft 

 layers, and an accumulation of clay from such layers in places where the 

 pressure is less strong. This postulates a gentle seaward flow of layers 



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