THE TUNNEL THROUGH MONT CENIS. 169 



neers of this gigantic work have had to contend. A 

 large part of the rock consists of a crystallized cal- 

 careous schist, much broken and contorted; and 

 through this rock run in every direction large masses 

 of pure quartz. It will be conceived how difficult the 

 work has been of piercing through so diversified a siib- 

 stance as this. The perforating machines are calculated 

 to work best when the resistance is uniform ; and it 

 has often happened that the unequal resistance offered 

 to the perforators has resulted in injury to the chisels. 

 But before the work of perforating began, enormous 

 difficulties had to be contended with. It will be un- 

 derstood that, in a tunnel of such vast length, it was 

 absolutely necessary that the perforating processes 

 carried on from the two ends should be directed with 

 the most perfect accuracy. It has often happened in 

 short tunnels that a want of perfect coincidence has 

 existed between the two halves of the work, and the tun- 

 nellers from one end have sometimes altogether failed 

 to meet those from the other. But in a short tunnel 

 this want of coincidence is not very important, because 

 the two interior ends of the tunnellings cannot in any 

 case be far removed from each other. But in the case 

 of the Mont Cenis tunnel any inaccuracy in the direc- 

 tion of the two tunnellings would have been fatal to 

 the success of the work, since when the two ought to 

 meet it might be found that they were laterally sepa- 

 rated by two or three hundred yards. Hence it was 



