356 A. C. LANE WEIGHT OF SEDIMENTARY ROCKS 



gravity, from which the volume weight can be calculated. It was, how- 

 ever, pointed out in the discussion that the lumps he used, being less 

 friable, were probably more cemented and less porous than the average. 



Yet while the mineral specific gravity was almost invariably between 

 2.6 and 2.7, the pore space ran as high as 37.7 per cent for the Tertiary 

 sands of Mexico and 36.7 for those of Louisiana. The Paleozoic sands of 

 Pennsylvania were naturally less porous, 18 per cent being near the maxi- 

 mum porosity. This would mean 1,800 ounces and 2,200 ounces per 

 cubic foot respectively. 



Methods of determining Weight per unit Volume 



The weight per unit volume may be determined from the specific weight 

 of the rock substance and the porosity. This is, however, rather cum- 

 brous, and it seemed well to have more direct and authentic ways of 

 measuring this weight, which, while less accurate, perhaps, in some cases 

 were simpler, more rapid, and more authentic in others. 



There are three ways in which I have done this : 



1. In the first place, I have taken cylinders and measured their volume 

 and weighed them. This is the method which was successfully used by 

 Sidney Powers and myself 8 in tracing the variation in weight due to 

 magma tic differentiation in a series of diamond-drill cores from a 500- 

 foot trap sheet. For hard rocks samples have been diamond-drill cores, 

 but with an instrument like Davis's peat sampler it is quite possible to 

 take short cylinders of clay and to measure their volume with an accuracy 

 near 1 per cent. 



2. A second method is to cut out bricks, though I find that to get sharp 

 forms they have to be rubbed down to a smooth surface, perhaps on sand- 

 paper or a file. 



3. A third method, which I have recently used at the hint of A. F. 

 Melcher, 9 is in some ways more convenient, is capable of greater accuracy, 

 and is particularly applicable to friable material. 



A mass of friable material, which should be cool, is dipped in paraffin 

 just melted; a little unmelted piece floating about is desirable. This 

 makes a solid mass, the weight of which in water can easily be obtained, 

 and by simply taking the weight before dipping, after dipping, and in 



8 Am. Inst. Min. Eng. Bulletin, 1916, pp. 534-548, 893; Transactions, vol. liv. 1917. 

 p. 450. 



9 A. F. Melcher : Determination of pore space. Transactions American Institute of 

 Mining and Metallurgical Engineers, 1921. vol. Ixv. p. 470. Reference is made to 

 Hirscbwald's "Die Priifung der Baiisteine auf ihre Wetter bestandigkeit" (Berlin, 1908, 

 Ernst und Sohn), but I have not seen that. See also Roscnbuscb Wiilfiug, "Mikrosko- 

 piscbe Pbysiographie," vol. i, 1904, part 1, p. 46. 



