1919.] 
Departmental Eeports. 
389 
As was pointed out in Geological Survey Bulletin No. 17 (page 142), 
Arber’s explanation is not applicable to the Mangatini-Matipo split. The 
angle of divergence is too great to be explained by such earth-movements 
as he postulates, and, moreover, no evidence of contemporaneous tilting 
of the strata to the north or south can be observed. There is, however, 
another explanation of the divergence of splits which has its basis in the 
diminution of bulk that takes place in vegetal matter during its passage 
into coal. This has been estimated as varying from nine-tenths to nineteen- 
twentieths the original volume of the vegetal matter. Obviously the ratio 
is not fixed, but varies according to the character of the material compressed 
and the chemical changes that take place in it during its alteration into 
coaly substance. In the case of mud consolidating to a shale the diminution 
of bulk, chiefly owing to the expulsion of water, is considerable, but in the 
case of sand or grit the change of volume is small. Hence a 10 ft. coal- 
seam, representing from 100 ft. to 200 ft. of vegetal matter, can elsewhere 
be represented by 100 ft. or more of a rock such as sandstone, and conse¬ 
quently there is no difficulty in explaining the divergence of the two parts 
of the Mangatini-Matipo seam as due to the deposition of mud, sand, and 
grit in place of vegetal matter. It will be observed that, as the space 
between the split increases, the amount of coal present in the section 
decreases, and, although the rate of variation is not uniform, this cannot 
be held to invalidate the explanation, for, as is-well known, New Zealand 
coal-seams vary in thickness to a remarkable degree. 
On the assumption that the normal thickness of the Mangatini-Matipo 
seam in the area near T 31 Creek is 25 ft., it represents from 250 ft. to 
500 ft. of vegetal matter. It is therefore possible for its splits to diverge 
by at least 250 ft. without there being any necessity to invoke earth- 
movements as the cause of splitting. The actual divergences of the two 
splits, together with other data, are shown in the following table. The 
normal total thickness of coal in the section is assumed to be 25 ft., and the 
average dip of the strata to be 18°. Hence all thicknesses found by boring 
are diminished by multiplying them by 0-951 (natural cosine of 18°). 
(1) 
No. of Bore. 
(2) 
Apparent Diminution 
of Thickness of Coal 
in Feet. 
(3) 
Thickness of Wedge 
of Rock in Feet. 
(4) 
Ratio of (2) to (3). 
6e (or 7) 
4-9 
99-9 
1 to 20-4 
7e . . 
12-3 
104-6 
1 to 8-5 
4e .. 
15-3 
111-3 
1 to 7-3 
1 .. 
12-9 
133-1 
1 to 10-3 
Average 
11-35 
111-2 
1 to 10 (nearly). 
From the above table it will be seen that at bore 6e the ratio* of the 
apparent coal-diminution to the thickness of strata in the rock-wedge is 
much greater than would be expected, but this can be accounted for as due 
to a local thickening of the lower split. On the other hand, at bores 7e, 4e, 
and 1, the ratio shows an approach to what might be expected if allowance 
for some shrinkage in the lower layers of the vegetal deposit prior to 
burial is made. The data need not be further discussed, for it is clear that 
the Mangatini-Matipo split can be satisfactorily explained as caused by an 
* This ratio, expressed as a number, may be called the “ change-of-volume factor.” 
See the subsequent article entitled “ The Application of a Change-of-volume Factor to 
the Correlation of Coal-seams and Coal-bearing Strata.” 
