Since Mr. Millard prepared this paper, the ideological Survey Department has published 
a later set of analyses.* In order to check Mr. Millard’s isovolves, which is a very important 
matter. I have found it necessary to .work out some hundreds of carbon ratios, not only from 
the analyses given in the bulletin referred to. but from an earlier edition, dated 1921. which gives 
analyses from different localities in some cases. In consequence, 1 find it necessary to criticize 
somewhat Mr. Millard’s results in spite of the fact that 1 consider the lines upon which he has been 
working to be very sound indeed. 1 doubt whether it is possible to draw the 60 isovolves through 
the Newcastle Coal-field to the east of Maitland as shown. Out of over 70 analyses of coal from 
the Tipper Coal Measures taken from this area, only eight gave carbon ratios that were not in excess 
of 60. They varied between 57.90 and 64.98. Moreover, in five cases out of these eight analyses 
of coals from the same pit gave results in excess of 60, showing that in these cases the low' results 
were due to local causes. Even in the case of the lowest figure, which is from the Rhonda Colliery, 
and from a coal which the Geological Survey classes as inferior, a second analysis from the same 
pit gives a carbon ratio in excess of 60. Thus, only three results can be taken as lower than 60. 
and each of these is above 59. Moreover, collieries working the same seams on every side of them 
give higher results. The average carbon ratio for the Upper Coal Measures in the Newcastle- 
Maitland Coalfield area, w orked out from over 70 coal analyses, is 61.5. The Middle or Tomago 
Coal Measures give an average, from 15 analyses, of 60.5. although a higher degree of metamor- 
phism might perhaps be expected in these lower beds. 
Turning for a moment to the western rim of the Sydney Basin, the carbon ratio averaged 
from 50 analyses of coals taken from the Upper Measures is 63.90, while in the Southern Coal-field 
area, extending from Sydney to the southern margin, the average carbon ratio, from 40 samples 
of coals taken from the Upper Coal Measures, reaches as high as 73.68 an individual sample going 
up to 84.52. In this area, however, the effect of volcanic intrusives has played a great part in 
cindering the coals and causing metamorphic changes to take place in the strata. The conclusion 
I come to from these figures is that the 60 isovolve cannot be drawn through the Sydney Basin, 
although there are small areas in the north-west corner in the neighbourhood of Singleton and 
Muswellbrook, wliere the carbon ratios in the Upper Coal Measures appear to average low r er than 
60. Thus the 60 isovolve may only he drawn to include small and relatively unimportant areas, 
but it cannot be represented as a line taking a general course through points in the Basin. There 
is apparently a progressive increase in metamorphism as we pass from the Hunter River to the 
south and south-west, but the basis from which it proceeds is metamorphism which is represented 
hv a carbon ratio of over 60. This conclusion may seem rather remarkable at first sight, for Mr. 
Millard’s isovolves for the Greta Coal Measures are substantially correct. It would appear then 
that there must be some progressive change between the 50 and 55 isovolves of the Greta Coal 
Measure areas and the Coal Measure areas around them. But here we meet a new' problem : 
There are possibly as much as 7.000 feet of sediments between the Upper Coal Measures which 
we have been considering, and from which the foregoing results were obtained, and the Greta 
Beds. Can we assume that the degree of change which has taken place and has been traced in 
one set of beds has gone on to the same degree exactly in another set of beds wdiicb lie 7.000 feet 
below ? That is to say, if the Greta seams covered the whole, floor of the Sydney Basin, can we 
assume that the carbon ratios in the Greta stage would coincide with those of the beds above on 
the western and southern margins ? That would be too much to assume in the present state of 
our knowledge. When we come to the Greta Coal Measures 1 think we enter upon a new problem, 
and that the carbon ratios of the Greta coals are a distinctly separate problem from those of the 
overlying Goal Measures. In other words, we complete one problem when we deal with the carbon 
ratios of the Upper Beds, and must start again when we consider the Greta coals. 
It has been loosely stated that the carbon ratios for the Greta coals are higher than for 
the overlying coals. It is quite otherwise. From 70 analyses made of the Greta coals, the average 
carbon ratio works out at 55.5. This fact alone proves that little can be assumed in such an 
investigation. However, it causes some difficulty since the carbon ratio is taken to represent 
the degree of metamorphism which has taken place in the strata, and one would naturally expect 
to find this greater in beds so much lower in the sequence than in the higher ones. Mr. Millard 
explains this phenomenon by the following reasoning : We know that uplift was taking place 
in the Lochinvar area in pre-Permo-Carboniferoiis times, and the uplifted area may have reached 
such a level that it formed high land during the period of deposition of most of the upper coal- 
bearing sediments which were never in consequence deposited above the Greta coals in this area. 
Thus these coals have suffered less metamorphism. because the pressure of overlying sediments 
was not very considerable. Professor David and the officers of the Geological Survey do not 
agree with this view. In fact, Professor David states " The extra hardness of the Greta is partly 
to he accounted for by the fact that it has been under the pressure of an additional 7,000 or 8,000 
* “ The Coal ^Resources of New South Wales " -Bulletin No. 6, dated 1024. In the only copy available, winch is a proof ropy, it is ro dated, but it was 
not completed for issue til! 1925. 
