L^ECEMBER 7, 1917] 



SCIENCE 



555 



from that of salt. There is a real possibility, 

 however, that the difference is great enough 

 so that large bodies of rock salt not far from 

 the surface can be detected by determinations 

 of the intensity of gravity. 



While investigating the " mud lumps " at 

 the mouths of the Mississippi a few years ago 

 the writer had occasion to study isostacy a little, 

 for it seemed probable that the " mud lumps " 

 were due to gravity — induced internal flowage 

 of the delta. The question arose, may not the 

 salt domes be due to some such solid or semi- 

 solid flowage? and another question immedi- 

 ately arose, namely, may not the domes have a 

 perceptible effect on gravity? The domes of 

 southern Texas and Louisiana are in a region 

 that is very flat, and although some domes are 

 marked by knolls from two to three to thirty 

 feet or more in height, some domes that are 

 very high structurally have little or no effect 

 on the surface. Whether this indicates that 

 many domes are antecedent to the surface de- 

 posits has not been determined. In any case 

 the means now available for finding domes not 

 marked by a hill or basin on the surface seem 

 to be limited to scant and irregularly devel- 

 oped secondary deposits at the surface, such 

 as the curious " paraffin earth " which is ap- 

 parently a new compound, though containing 

 possibly both gelatinous silica and some hydro- 

 carbon. 



Since the domes are in a flat region under- 

 lain by comparatively homogenous sand, silt 

 and clay, it seems more than likely that the 

 salt, dolomite, gypsum, sulphur, compressed 

 clay and possibly igneous rock of the domes 

 would together have a specific gravity notice- 

 ably different from that of the country rock in 

 which they occur, and within the range of 

 possibility that the difference could be de- 

 tected by gravity observations. In other 

 words it seems possible that hidden salt domes, 

 with the immensely valuable pools of oil and 

 gas that are commonly associated, can be dis- 

 covered through the help of gravity observa- 

 tions, which will thus reduce to a greater or 

 less extent the cost of finding the oil pools. 



The intensity of gravity varies with altitude, 

 latitude, topography and the varying density 



of the materials composing the earth, partic- 

 ularly near the points where the observations 

 are made. 



A mass weighing 200 povmds at sea level at the 

 equator will weigh [on a spring balance] approxi- 

 mately 201 pounds at sea level at either pole. A 

 mass weighing 400 pounds at sea level will weigh 

 approximately 399 pounds at an elevation of 5 

 miles at the same latitude; and a given mass will 

 weigh less at the top of a sharp mountain peak 

 than if it were at the center of a broad plateau of 

 the same elevation as the peak. . . . The measure- 

 ment of the force of gravity at a station to be ac- 

 ceptable must not have a probable error greater 

 than one part in two hundred thousand. An actual 

 error of one part in two hundred thousand corre- 

 sponds to an error of only one one-millionth of a 

 second in the period of oscillation of the pendu- 

 limi.i 



The method of observation, consists essen- 

 tially in determining the effect of gravity on 

 the rate of swing of a pendulum. The instru- 

 ment " is placed on a solid concrete floor or an 

 especially prepared brick or concrete pier." 

 With the interferometer, vibrations of the case 

 " due to the passing of a team a city block 

 away or a team a mile away are easily de- 

 tected." 



The average probable error in the gravity 

 observations of the Coast and Geodetic Survey 

 is said to be in general about .002 or .003 of a 

 dyne. 



If we assume that the force of gravity at the cen- 

 ter of a section [square mile] which is underlain 

 with sand one mile deep at a specific gravity of 

 2.50 is 980,000 dynes, then if the [cubic mile of] 

 sand were replaced by limestone at a specific grav- 

 ity of 2.75 the force of gravity would be increased 

 to about 980.005 dynes. Similarly, if the same 

 were replaced by basalt at a specific gravity of 

 3.00 the force of gravity would be increased to 

 about 980.010 dynes. The change in the force of 

 gravity at the center of the adjacent section due to 

 these changes in specific gravity would be about 

 one fifth as much as in the section affected.2 



Apparently, if a cubic mile of clay and sand 

 with a specific gravity of 1.80-2.00 immediately 



1 U. S. Coast and Geod. Survey, Spec. Pub. No. 

 23, pp. 48 and 50, 1916. 



2 Letter to writer from acting superintendent 

 Coast and Geod. Survey, April 4, 1916. 



