At Mount Kembla the erosion has cut partially through 

 the main coal seam and has very seriously reduced its 

 workable thickness. In the more northerly collieries the 

 coal is only locally affected by washouts, and there is 

 usually a thin but variable bed of shale between the coal 

 and the sandstone. 



The Bulli Seam has an average thickness of about 7 feet 

 in the southern part of the district. It is a strongly 

 laminated, hard steam coal. It is somewhat jointed, and 

 for the most part these " backs " are vertical. The coal 

 is somewhat porous, some laminae more so than others, a 

 distinction which is brought out in the older mine workings 

 by the selective growth of mould on the damp bands. 



The floor of the Bulli Seam is a dense fine-grained shale 

 of very uniform character throughout, quite similar to the 

 Wianamatta Shale of the Sydney area. It is apparently 

 very impervious to moisture. 



The four foot sandstone, so called because it forms the 

 roof of the four foot seam, at Mount Keira is upwards of 

 30 feet in thickness, but a little further north it thins down 

 to 10 feet. 



The " stone rolls " take the form of long ridges of the 

 floor shale of the Bulli Seam with a fairly constant trend 

 about N. 40° VV. to S. 40° B. They are not perfectly straight 

 and in some cases swing round in a direction 20' nearer 

 the meridian. The average strike of the axes of the rolls 

 is parallel to a well defined axis of weakness in the district, 

 since the faults which occur at intervals trend in about the 

 same direction. In addition to the main rolls there are 

 occasionally encountered rolls at right angles to them, 

 called locally "tie rolls." The occurrence of the rolls is 

 rather limited, they appear to extend from Mount Kembla 

 on the south to North Bulli on the north. 



