244 COALFIELDS AND COLLIERIES OF AUSTRALIA. 



the face of the headings at distances not exceeding 20 yards. 

 The A. A. Company has decided not to work coal under a 

 less cover than 140ft., while the Stockton and Hettoii col- 

 lieries have a thickness of rock overhead of 250 and 300ft. 

 respectively. The boreholes are generally made by a machine 

 with a serrated steel bit, which gives a lin. core. Boreholes 

 30ft. long generally cost about 2s. per foot. Where the 

 roof is bad and the coal thick and tender, leaving some coal 

 next to the roof has been found to have a strengthening effect. 



On account of the unconsolidated alluvial material, often 

 heavily charged with water, found near the surface at the 

 site of certain shafts, such as at the Hetton, Wickham, and 

 Bullock Island, and Stockton collieries, the ground had to be 

 secured by means of. cast-iron tubbing. In the case of the 

 Stockton No. 3 shaft, this tubbing was continued to a depth 

 of 281ft. 



The greatest distance driven under the ocean off the New 

 South Wales coast up to date for coal winning purposes is from 

 48 to 50 chains. It is most important that the sea should 

 not be allowed to break into a colliery, for not only would 

 the workings of that particular colliery be flooded, but those 

 of any adjoining colliery to the dip which might be connected 

 with it. Such an accident did actually occur on a compara- 

 tively small scale on 18th March, 1886, when the Ferndale 

 colliery and about 20 adjacent small collieries were inundated 

 with sea water and sand from a tidal stream named Tighe's 

 Creek, and adjoining swamp, and irretrievably lost. 



The Wallarah Colliery. 



This colliery, situated at Catherine Hill Bay, is the 

 southernmost colliery of the Northern coal field. Other pro- 

 perties have been taken up still further south, but, so far, only 

 boring operations have been carried out on them. Originally 

 this property belonged to another company, who worked a 

 lower seam than that now operated on by the present com- 

 pany. The seam was attacked from a mine a few yards 

 south of the jetty, but being dirty coal, it was eventually 

 abandoned. The Wallarah Coal Co., an English corporation, 

 took the property over about nineteen years ago. Mr. Joseph 

 Sperring came out from England at that time, and acted as 

 under manager till ten or eleven years ago, when he was 

 appointed manager. The seam at present being worked aver- 

 ages about 12ft. Gin. thick, but only 6ft. of the bottom coal is 

 worked. Both the top coal and the rock above make a good 

 roof. 



Two districts, known respectively as the "B" and "E," 

 are now being worked. Both are approached by 



