'^66 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 14, 



Jimction of Coal and Granite, Blackheath Mines, -■' '-- 





and the forcing of the granite against the coal, the distinct layers of 

 which are often cut off abruptly one after the other by the granite in 

 contact. The dips of the strata of coal in the neighbourhood of these 

 sudden thickenings and swellings of the seams vary from 20° to 70°, 

 and at one point attained an angle of 84°. Polished surfaces or slicken- 

 sicles, with their planes turned in various directions, also bear testi- 

 mony to the sliding and friction which the beds have undergone. 

 Some of the alleged varieties in the thickness of the great seam at 

 short distances may be explained by sudden changes in the angle of 

 dip, which, as before stated, might cause an exaggerated estimate 

 wherever an inclined stratum is cut through vertically in vertical 

 shafts ; but it appears that where the coal is nearly horizontal, it often 

 is far from miiform in strength at points a few hundred yards apart. 

 The beds which I actually observed at Blackheath and the neigh- 

 bourhood varied in their inclination from 8° to 42°, the dip being 

 always Avesterly. 



The great uncertainty of the depths at which the coal can be won 

 in sinking shafts at known distances from the natural outcrop or from 

 previous workings, proves the irregularity of the dip, and implies in 

 some places sudden flexures, or more probably vertical faults. An 

 example of one of these occurs in comparing the sections exposed in 

 the Creek and White Chimney pits. A shaft at the former or Creek 

 mine being sunk 400 feet deep to the main seam, the coal was fol- 

 lowed for 1 5 yards westward in the direction of the dip at an angle of 

 20°. The White Chimney or Old Midlothian shaft was then sunk 

 500 yards westward of the former, and the same great bed of coal, 

 with the granite below it, was entered at the depth of 360 feet. If 

 the dip of about one in three continued westward, in this instance, an 

 upthrow of more than 100 yards would be required to account for the 

 position of the coal so near the surface m the more westerly pit. 



About five miles north of the Midlothian shaft last mentioned, at 

 Duval's pit, which is also on the eastern outcrop, the coal-measures are 

 seen near the granite to be arranged in a small trough extendmg only 

 a few hundred yards east and west. At this place there are three beds 

 of coal, which in the deepest part of the small basin are only 200 feet 

 from the surface. Again, at the northern extremity of the whole coal- 

 field, at the Deep-run pits, there is another example of a small basin 

 only a mile and a half in its longest diameter, from N.N.E. to S.S.W., 



