246 



is known to occur in this series in the neighbourhood of 

 Sydney harbour though a few miles to the west and in 

 other areas of the Carboniferous, gypsum forms an im- 

 portant member of the series. 



The Millstone Grit is largely composed of coarse and 

 fine, grey or green sandstones in part conglomeratic 

 especially towards the base of the series, and shales usually 

 dark in colour. In the eastern part of the field shales 

 are relatively more abundant, are more largely red, and at 

 least one important coal seam is present, whereas, to the 

 west the shales are generally dark, are less abundant, the 

 conglomeratic phases of the sandstone are more prominent 

 and coal seams are absent or relatively unimportant. 



The Productive Coal Measures are largely shales, 

 commonly dark coloured but also in part red or green, 

 and light coloured sandstones. Thin persistent beds of 

 dark limestone form a characteristic feature of the lower 

 portion of the division. In various measured sections, 

 there is, on an average, 24 coal seams with a total average 

 thickness of 46 feet (14 m.) of coal. 



The thickness of the three lower divisions varies from 

 place to place. At the eastern end of the field, on the 

 shores of Mira bay, the Millstone Grit has an estimated 

 thickness of about 5,700 feet (1,740 m.) ; on Sydney 

 harbour, the measured thickness is 3,625 feet (1,105 rn-) ! 

 while farther west, the thickness decreases to about 2,000 

 feet (610 m.). The Limestone series shows a more marked 

 variation in thickness, ranging from 4,000 feet (1,220 m.) 

 or more at the foot of Sydney harbour to less than 900 feet 

 (275 m.) on the east side of George river only 4 miles 

 (6-4 km.) to the west. The Conglomerate series exhibits 

 a still wider range of thickness, since in places it is altogether 

 wanting. 



Of the great volume of Carboniferous strata, by far 

 the greater part is apparently of continental origin and 

 practically only in the case of the Limestone series with 

 its fossiliferous limestones and shales, is there positive 

 evidence of normal marine origin of any of the strata. 

 The evidence found in the Productive Coal Measures of the 

 former existence in situ of forest growth; the abundant 

 plant remains found in the Millstone Grit and their occur- 

 rence in a fragmentary state in the Conglomerate series; 

 the character of, and the channeling phenomena, etc. 

 exhibited by the sandstone beds that form so large a part 



