| Name. | Material. | Cost per cubic yard. |
|---|---|---|
| Blackrock, U. S. | hard greywacke slate, | $6.60 |
| Lehigh, U. S. | very hard granite, | 4.36 |
| Schuylkill, U. S. | slate, | 2.00 |
| Union, U. S. | slate, | 2.08½ |
| Blue Ridge, U. S. | ——, | 4.00 |
The Blue Ridge tunnel on the Virginia Central Railroad is 4,280 feet long, made for a single track, 21 × 15 feet. Lining about four feet thick. Excavation where lining is used is 26 × 23.
The Hoosac tunnel (Massachusetts) is proposed to be four and one half miles long, 23 × 22 feet section. To have two shafts eight hundred and fifty and seven hundred and fifty feet deep, and ten feet in diameter.
Artificial ventilation becomes necessary in headings over four hundred and fifty or five hundred feet in length.
The cost of the shafts of the Blechingly tunnel, (England,) ninety-seven feet deep, and ten and one half feet in diameter, cut through blue clay, and lined, was $68.44 per yard down.
The shafts of the Blaisy tunnel average five hundred feet deep, through clay and chalk and loose earth, (being lined,) cost $139.11 per yard down.
The shafts of the Black Rock tunnel, one hundred and thirty-nine feet deep, in hard slate, cost $18.72 per cubic yard.
CHAPTER VIII.
WOODEN BRIDGES.
139. Wooden bridging, owing to its cheapness and fitness for universal application, has been and is being adopted in all parts of the country. Almost any variety of form may be seen upon our railroads, and though less durable than stone or iron, it may with proper precaution be made to last a long time.