MANNER OF EXCAVATION. 



"bottom green-earth, and leave it naked, for the extent of some 70 or 

 80 feet length of the lower end of the range (r, in Fig. III.). This 

 completing of the excavation through the lowest marl was begun 

 at the lower end, where is the outlet for water from the pit. , And 

 as soon as each successive few yards in length had been so deepened 

 to the bottom, the side-wall of marl was cut down, and its marl 

 mostly saved. Then, indeed, the earth, which this wall had been 

 left as a barrier to keep out, fell in, and more or less of the old con- 

 fined water beneath the earth flowed in also, from the old diggings. 

 But this now did no harm. The bottom (green-earth) where the 

 caving earth fell, and the water overflowed, was not needed ; and 

 the water, after rising a few inches thereupon^ passed off through 

 the outlet into the stream. The next few yards length of bottom 

 marl was then removed, and then its part of the side-wall taken 

 down in like manner, until the whole lower 70 or 80 feet (z, r, 

 in Fig. III.) had been taken out, including its adjoining'side-wall. 

 Next, the lowest part of the sloping roadway of marl (#, r, Fig. 

 III.) was dug out, the carts turning and loading on the adjoining 

 space next above, which also was next dug out. Thus, the whole 

 slope of marl was dug out in the manner of successive perpendicu- 

 lar pits (v, v, v, &c.), each of the full width of the range, and 6 

 or 8 feet in the direction of the range. The first or lowest of these 

 perpendicular diggings was not a foot deep at the upper side. Each 

 increased in thickness, until the last and highest (0, Fig. III.) at be- 

 ginning of the slope of the overlay forming the road, was 8 feet thick. 

 Still, though this digging was according to the former mode, by 

 perpendicular pits, this operation was much less laborious ; as the 

 throwing up of the marl varied in height from less than one foot, 

 to at most but 8. As fast as each of these pits was finished to the 

 bottom, the adjoining part of the side-wall (o, Fig. IV.) was cut 

 down, and as much saved as was not prevented by the coming in 

 of earth and water. 



The thinner part of the overlay forming the slope (m, o, Fig. 

 III.) was next thrown, in successive uncoverings, into the last 

 finished pits, and the marl below in like manner dug out. But 

 when the thickness of this earth had reached some 3 feet, and about 

 50 feet of the length remained, this remnant was left to be taken 

 out with the next succeeding range of marl ; when carts, by having 

 a longer route, could descend on this part of the marl by a slope 

 made in a reverse direction (as space h o h, in ranges E and F, 

 Fig. II.) This postponement of the complete uncovering and the 

 digging out of the marl of the upper 50 feet of each range, for the 

 benefit of more easy access at a later time, has been since continued. 

 It is including all these more costly labours with the principal and 

 usual operation, that the excavation is deemed reduced in cost fully 

 one-half of that of the former mode. 



