330 



THE SUMMIT TUNNEL. 



CHAP. XVI. 



ENTRANCE TO THE BOMM1T TUNNEL, LITTXEBOROOGH. 

 [By Percival Skelton.] 



accident," about which so much alarm had been spread. 

 All that was visible was a certain unevenness of the 

 ground, which had been forced up by the invert under 

 it giving way ; thus the ballast had been loosened, the 

 drain running along the centre of the road had been 

 displaced, and small pools of water stood about. But 

 the whole of the walls and the roof were still as perfect 

 as at any other part of the tunnel. Mr. Stephenson 

 explained the cause of the accident : the blue shale, he 

 said, through which the excavation passed at that point, 

 was considered so hard and firm, as to render it un- 

 necessary to build the invert very strong there. But 

 shale is always a deceptive material. Subjected to the 

 influence of the atmosphere, it gives but a treacherous 

 support. In this case, falling away like quicklime, it 

 had left the lip of the invert alone to support the 

 pressure of the arch above, and hence its springing 

 inwards and upwards. Mr. Stephenson directed the 



