Institution of Civil Engineers. 403 



models that had recently been furnished by engineers to the Museum 

 of (Economic Geology. 



He then proceeded to remark upon the geological features of the 

 South- Western Coal-Field near Bristol and Bath, which had been 

 described by Mr. Conybeare and himself, in the Transactions of the 

 Geological Society of London (1824). 



Some of the tunnels near Bristol are driven in the Pennant grit 

 of the coal formation, where it is thrown up at a considerable angle, 

 and composed of strata yielding slabs and blocks of hard sandstone 

 used extensively for pavement. 



In traversing such inclined and dislocated strata, the engineer's 

 attention should, he conceived, be especially directed to the original 

 joints that intersect the beds nearly at right angles to their planes of 

 stratification, and also to the fractures produced during the move- 

 ments they have undergone. These natural divisions and partings 

 render such inclined stratified rocks unworthy of confidence in the 

 roof of any large tunnel, and liable to have masses suddenly de- 

 tached. 



Inclined strata of a similar sandstone are perforated by many tun- 

 nels on the railway near Liege, in nearly all of which the roofs are 

 supported by brick arches. 



It has been found impossible to make the tunnels through lias 

 and red marl without continuous arches of masonry. 



In any of the tunnels which have been carried through strata of 

 the great oolite, the parts left unsupported by masonry would, in his 

 opinion, be peculiarly liable to danger, because even the most com- 

 pact beds of oolite are intersected at irregular intervals by loose joints 

 at right angles to the planes of the strata, and occasionally by open 

 cracks : and it is to be feared that the vibration caused by the rail- 

 way carriages would tend eventually to loosen and detach these 

 masses of stone. ' 



He apprehended still greater danger would exist in tunnels cut 

 through the loosely joined strata of chalk, unless they are lined 

 throughout with strong masonry ; and even that, in a recent case, had 

 been burst through by the weight of the incumbent loose chalk 

 coming suddenly upon the arch. 



In open cuttings through chalk, where the numerous interstices and 

 the absence of alternating clay-beds prevent any accumulation of 

 water, there is little chance of such frequent landslips as occur where 

 beds of stone, gravel, or sand rest on beds of clay ; but until the side 

 walls of chalk are reduced to a slope at which grass will grow, they 

 will be subject to continual crumblings and the falling down of small 

 fragments, severed by the continual expansion and contraction of the 

 chalk, under the destructive force of atmospheric agents, and chiefly 

 of frost. 



In open cuttings, where the inclination of the strata is towards the 

 line of rails, the slope should be made at a greater angle than if the 

 strata incline from the rails ; if this be done, fewer landslips will 

 occur from accumulations of water between the strata thus inclined 

 towards the rails ; and such sjips may be further guarded against by 



