Chap. 7] GRAVITATIONAL METHODS 71 



body are known from drilling or from other geophysical surveys, or these 

 measurements may be made from a gravimeter traverse across a known 

 topographic feature not associated with structure/ The prevalent pro- 

 cedure is to secure representative samples from outcrops, well cuttings, or 

 ♦underground workings, and to test them in the laboratory. Methods of 

 rock-density determination do not differ much from standard physical 

 methods of measuring densities of solids. Difficulties arise only with 

 specimens from unconsolidated formations or Math samples of large pore 

 volumes or permeabilities. 



1. Measurement of weight and volume is widely used for determining 

 densities of surface strata in connection with torsion balance terrain correc- 

 tions and is practically the only method available for this purpose. A 

 comparatively large quantity of the sample is placed in a cylinder ranging 

 in volume from 1000 to 2000 cc, taking precautions not to alter its pore 

 volume. Cylinder and sample are weighed, and the weight of the empty 

 cylinder is deducted. If mass of the sample is m grams and its volume 

 is V cc, the density 8 = m/v. If a great number of determinations have 

 to be made, it is of advantage to use a portable balance with horizontal 

 arm graduated in density units, since the volume of the sample may be 

 kept constant. If the rock sample is solid and of irregular shape, its 

 volume is determined from the amount of water it displaces in a calibrated 

 glass cylinder; for water-soluble samples, alcohol, machine oil, kerosene, 

 toluol, or a saturated solution of the same substance is substituted. Air 

 bubbles must be removed with a brush, by shaking the vessel, boiling the 

 water, or by using an air pump. 



2. The pyknometer is a small glass flask of precisely determined volume 

 with a ground-in glass stopper extending into a fine capillary to provide 

 an overflow for excessive liquid. It is useful for measuring densities of 

 small specimens only. Determination of three weights is necessary: (1) of 

 the specimen (?n) ; (2) of the pyknometer filled with water (Q) ; (3) of the 

 pyknometer with specimen (Q'). Then the density of the sample: 5 = 

 m./{Q + m - Q'). 



3. Weighing in air and water is a very common method of density deter- 

 mination. If the weight of a specimen in air is Wo and under water it is m', 

 the buoyancy B = mo — m', and the density 8 = mo/B. An ordinary 

 balance may be adapted to this test by first balancing the sample on the 

 balance in air and then suspending it in water with a fine wire or in a 

 small pan. Allowance for the weight of this pan under water is made by a 

 corresponding adjustment of the balance. Specimens soluble in water 

 are weighed in some other liquid (see paragraph No. 1); the result is 

 multiplied by the specific gravity of the liquid used. Powdered minerals 



1 This applies only to near -surface formations. See L. L. Nettleton, Geophysics, 

 4(3), 176-183 (1938). 



