358 A. C. LANE WEIGHT OF SEDIMENTARY ROCKS 



taking 1 W p and subtracting it from W w p , we have 



W w and there is O.i Wp left — a small quantity ; 

 whence we have, neglecting it, 15.5 / (15.5 — T.3 = 8.2) 

 G w = W w / W w — W w p 8 (— O.i W p ) 



= 1.89 

 correction for paraffin .03 



1.92 



The error incurred by neglecting the last term in the denominator is 

 (if the weight of paraffin is not Over one-tenth of the other weights) less 

 than 1 per cent. It is the only term in which the specific weight of the 

 paraffin comes in, so there is no great error there, and if we neglect the 

 paraffin entirely and simply divide the weight by the loss of weight when 

 dipped in paraffin and immersed in water, we make no great error if we 

 keep the paraffin skin small. 



Similarly we shall find, for the dry weight, 



D = W d / (W w 



— Ww p s — .i W 



x 



G d = W d /V d 





= w d /w d - 



-W pd s — .1 Wp 



and if the shrinkage 8 is 12 to 15 per cent 10 

 V d =(l — S) v w 

 (1 — 8) G d = D 



We see that there is, as shown in Mead's diagrams, a connection be- 

 tween weight wet (G w ), porosity (P w ), per cent of moisture M, and min- 

 eral specific gravity G m ; and if we assume G d and D to be the same (as 

 we probably may for coherent rocks), or if we have measured the shrink- 

 age S in any way, they, too, are connected. 



If we assume some definite value for the mineral specific gravity, as 

 Mead calls it, (G m ) — for example, 2.8 — then a simple straight-line dia- 

 gram (figure 1 ) shows the connection between porosity and wet and dry 

 weights, and the per cent of moisture is a hyperbolic curve. 



4. The older method, that used by C. C. Moore, 2 depends on obtaining 

 the mineral specific gravity and the porosity by measuring the moisture 

 absorbed, and is thus described by him (page 134) : 



"The thoroughly dried rock specimen is kept in vacuo (about 29 inches mer- 

 cury) for several hours, and then air-free water is slowly admitted into the 

 evacuated vessel until the specimens are completely under water. Air is after- 



10 H. Ries : Report on clay. Oeol. Survey Mich., vol. viii..part 1. pp. fi to 12. The 



30 to 40 per cent on page 12 is by volume. 



