352 RISKS OF GRADUATED PITS. 



Thus it appears, that if 100 bushels of marl had been carted to 

 12 J miles distance from the pit (making the trip to and fro 25 

 miles), the cost of carting would be $6.112, which added to the 

 other fixed expenses, 29.5 cents, shows the total cost to be $6.41 

 the. 100 bushels. 



Superior in general advantage as is the mode of working of marl 

 in large graduated excavations, it is very hazardous in wet situa- 

 tions, without much care. The liability to damage is especially 

 great when the work of an unfinished excavation is suspended 

 through winter. Then the caving in of the side-walls, both of 

 overlay and of the marl, caused by frequent rain floods, and still 

 more by the frequent alternate freezing and thawing of the exposed 

 marl, may operate first to cho^e the passage, and soon to crumble 

 down the entire side-drain. The outlet of water from the pit is 

 thus obstructed, and the quantity dammed up in the pit converts 

 the caved earth and marl to a mire. The successive freezing and 

 thawing continue to throw down successive layers of the walls, 

 serving still more to raise the water, and filling the pit with mire. 

 It has happened in my much earlier labours, elsewhere, that the 

 unfinished bottom of large spaces of marl was thus so covered in 

 deep mire, as to be given up, because not worth the great labour 

 of being again uncovered. 



The surest safeguard against such dangers is to complete the ex- 

 cavations of each such large digging before freezing weather; also 

 to throw in enough of the next overlying earth to cover the naked 

 upright wall of marl, and thus protect it from freezing. Then the 

 marl under the sloping roadways may be safely worked through 

 winter, in perpendicular pits, and each excavation, as soon as 

 finished, filled with earth, in uncovering another space of marl. 



But when the extent of the range, or the insufficiency of the 

 force compels the large excavation to remain unfinished through a 

 winter, other means may be used, varying according to the features 

 of each locality, to prevent much loss, and which will be suggested 

 by the peculiar circumstances to the mind of every observing 

 marler. 



In removing overlying earth, the excavation should not be 

 limited precisely to the laying naked a sufficient surface of marl, 

 and leaving the section of earth above nearly perpendicular. Even 

 if there is no likelihood of the earth so left caving down in masses, 

 and endangering the labourers below, the earth will be washed 

 down by every rain in small qantities; and crumbled down by 

 alternate freezing and thawing, if in winter. The face of the over- 

 lying earth should be cut to a slope (as seen in Fig. IY.) Then 

 if a layer is crumbled by freezing, or by drying, the loose 

 earth is kept in its place by its gravity. It is even cheaper, or 



