Two things are all-important in the choice between the steel bridge and the arch bridge of stone or concrete. The first is the accessibility of the quarries. If they are not very near the solid bridge will cost four times that of one of steel and the average American railroad is not able to spend money in that fashion, even in the hopes of future economies in maintenance. If the quarries are close at hand, as they were years ago when Kirkwood built the Starucca Viaduct for the Erie, the cost of a masonry bridge will hardly exceed that of steel trusses, and the concrete structure may cost a little less. Then there comes into play the second consideration. The stone or concrete bridge has tremendous weight, no ordinary foundation work will serve it. If the river bed and banks be of sand or poor earth, the engineer had best give up his hopes of the Roman form of structure. He can build steel towers and trusses on piles of caissons--hardly solid stone piers and abutments and aides. All these things considered, the stone bridge is still more than holding its own in modern railroad construction. The Boston & Albany Railroad began building these splendidly permanent structures along its lines through the Berkshires more than twenty years ago. More recently both the Pennsylvania and the Baltimore & Ohio have been looking with favor upon this type of bridge. The Baltimore & Ohio has just finished building its massive Brandywine Viaduct, near Wilmington, a splendid double-track structure, 764 feet in length, and composed of two 80-foot, two 90-foot, and three 100-foot arches. The three great stone bridges that the Pennsylvania has built upon its main line are all four-tracked. Two splendid examples of these span the Raritan River at New Brunswick, and the Delaware at Trenton, New Jersey. The third, spanning the Susquehanna at Rockville, Pa., just north of Harrisburg, is the largest stone bridge in the world. It is over a mile in length, and is composed of 48 arches; 220,000 tons of masonry was employed in its construction. Concrete viaducts were first employed in interurban electric railroad construction, and latterly they have been brought more to the service of the steam railroad. A splendid example of this very new form of construction exists in the extension of the Florida East Coast Railroad over the keys and shallow waters of Southern Florida, for seventy-five miles between Homestead and Key West. A considerable portion of the line is over the sea.