392 The N.Z. Journal of Science and Technology. [Dec. 
to use the term “ change-of-volume factor ” as equivalent to the term 
“ reciprocal of the shrinkage factor.” The change-of-volume factor, there¬ 
fore, is the average number of feet of vegetation required to form 1 ft. of 
coal. Ashley’s value of 3 is admittedly a minimum estimate, and is much 
too low to be correctly applied to the whole of a bed of peat or other 
vegetal matter. Even if his assumptions are accepted, the change-of- 
volume factor may be estimated as nearer 5 than 3. He assumed, more¬ 
over, a much smaller loss of vegetal matter through chemical changes in the 
formation of bituminous coal than most other writers. It therefore seems 
reasonable, on theoretical grounds, to suppose that the change-of-volume 
factor in most cases is not less than 6 or 7. 
Ashley attempted to calculate the change-of-volume factor from field 
data, and obtained the following results : (A) between 4 and 7 ; (B) 4 ; 
(C) 8§. He also quotes from W. T. G-riswold a case in which 1 ft. of coal 
seems to have been formed from 20 ft. of vegetal matter.* In the case 
of the Mangatini-Matipo split, Westport district, I have found the following 
values for the change-of-volume factor : 20-4, 8-5, 7*3, 10-3, and over 8. 
The next paragraph shows how a value of 7-4 can be deduced from data 
obtained in the Gfreymouth district. 
As an example of the correlation of coal-seams with the aid of a change- 
of-volume factor the logs of two bores quoted on page 128 of N.Z. 
Geological Survey Bulletin No. 13, 1911, may be taken. From these it 
appears that in bore No. 1, from the floor of seam 1a to the floor of the 
main or No. 1 seam, there is a thickness of 118 ft. 3 in. of strata, containing 
11 ft. 10 in. of coal, whilst at No. 4 bore, about three-quarters of a mile 
to the south, the thickness of the strata from the floor of a small seam, pro¬ 
bably 1a, to the floor of No. 1 seam is 154 ft. 8 in., including 6 ft. 2 in. of 
coal. The difference in the thickness of the stone strata is 42 ft. 1 in., and 
this divided by the difference in the thickness of the coal-beds—namely, 
5 ft. 8 in.—gives a factor of 7*4, which corresponds well with most of the 
determinations previously quoted. This result makes it tolerably certain 
that the thin seam found in No. 4 bore and tentatively correlated with 
the 1a seam is really that seam. This correlation then helps to make it 
clear that the base of the Island sandstone has been correctly determined 
in each bore. It is true, however, that when the method is applied to 
the correlation of the No. 2 seam in each bore the result is not consistent 
with the assumption tacitly made that the floors of all the seams were 
originally horizontal, and would be parallel if there were no shrinkage of 
vegetal matter. The discrepancy is not serious, and can easily enough 
be explained away. Another assumption made is that the shrinkage of 
the material forming the shale and other stony beds is much the same at 
both bores, so that no material error is introduced by neglecting it. 
Although the method of correlation outlined is far from being certain 
in application, it is believed that it will be useful in many cases, especially 
where the logs of bores drilled at no great distance from one another have 
to be compared. The change-of-volume factor of most general application 
is probably about 7 or 8. A somewhat lower factor may be assumed for 
brown coal and lignite, which presumably have not lost so much material by 
chemical change as bituminous coal, and in general have a slightly lower 
specific gravity. As already indicated, thick coal-seams call for a smaller 
factor than thin seams. 
* G. H. Ashley, loc . cit ., p. 45. 
