1919.] 
Departmental Reports. 
391 
of the intervening stone strata is to be found in the shrinkage of vegetal 
matter during its conversion into coal-seams of varying thickness. The 
hypothesis is, of course, not new, but hitherto does not seem to have received 
full recognition in the literature of coal,* and is probably seldom considered 
by those who have charge of coal-mines. In New Zealand, where the 
variability of coal-seams is far greater than in most European or American 
coalfields, opportunities for testing the correctness—or, rather, the utility— 
of the hypothesis are abundant. I therefore wish to bring it under the 
notice of those concerned in the development of our coalfields, and at the 
same time to show that it has a practical application in connection with the 
correlation of coal-seams, &c. 
The shrinkage of bulk in vegetal matter during its transformation into 
coal has been estimated as from 90 to 95 per cent.—that is, from 10 ft. to 
20 ft. of vegetation is required to form 1 ft. of coal. It is clear that if in one 
part of an area of sedimentation—say, on the margin of a lake—vegetation 
is accumulating, whilst contemporaneously in another part ordinary sediment 
is accumulating, approximately equal thicknesses of vegetal matter and of 
sediment may be formed. When, however, the vegetal matter has been 
transformed into coal, and the sediment into hard rock, the thicknesses 
will be very different. Though mud, as is a matter of common observation, 
shrinks considerably during consolidation, the change of volume is small 
compared with the change undergone by vegetal matter. Sand and similar 
material during consolidation undergo very little reduction of volume. 
Hence, in comparing one section of a coal-bearing series with another 
(contemporaneous) section, allowance should be made for the shrinkage of 
volume in the coal-forming material during its consolidation. The following 
rule may be enunciated : The more coal there is in a given section the less is 
its probable thickness as compared with a corresponding contemporaneous 
section containing little or no coal , and vice versa. 
Since in the case of a thick layer of vegetal matter the lower portion 
will be more compact than the upper, the number indicating the ratio of 
the volume of vegetal matter at the moment of burial to the volume of the 
coal produced will be variable. It will, of course, be larger for a thin layer 
of coal than for a thick seam. It will vary according to the nature of the 
vegetation, which may be wood, leaves, spores, grasses, reeds, or algae, or 
any conceivable mixture of all or any of these. It will vary also according 
to the amount of chemical change undergone by the vegetal matter during 
its transformation to coal. Moreover, the mode of accumulation, whether 
in situ or by transportation, will cause further variation. 
Ashleyf has estimated that 3J ft. of well-compressed peat will make 
1 ft. of bituminous coal such as that of the Appalachian field. Wishing 
to be conservative, he assumes that the factor 3 will represent the number 
of feet of well-compacted, deeply buried, old peat necessary to produce 
1 ft. of bituminous coal of the general character of the Pittsburg coal of 
Penusylvania. This number 3 is the reciprocal of what may be called 
the shrinkage factor, in this case For the sake of convenience I propose 
* See P. F. Kendall’s paper “ On the Splitting of Coal-seams by Partings of Dirt. 
Part I: Splits that Rejoin,” Trans. Inst. Min. Eng., vol. 54, p. 460, 1918. This 
has not been seen by me, but according to a review in the Geological Magazine, 
October, 1918, Professor Kendall gives prominence to the hypothesis mentioned above. 
See also the Colliery Guardian of 24th January, 1919, p. 189. 
t Gr. H. Ashley, The Maximum Rate of Deposition of Coal, Economic Geology , 
vol. 2, pp. 34-37, 1907 (see p. 41). 
