It is at least fifty or sixty miles long and runs along the base of the _Apennines_ in a line almost perfectly straight.

Does not its parallelism with the mountain chain suggest a causative relation?
See that other mighty _rill_, at least a hundred and fifty miles long, starting directly north of it and pursuing so true a course that it cleaves _Archimedes_ almost cleanly into two.

The nearer it lies to the mountain, as you perceive, the greater its width; as it recedes in either direction it grows narrower.
Does not everything point out to one great cause of their origin?
They are simple crevasses, like those so often noticed on Alpine glaciers, only that these tremendous cracks in the surface are produced by the shrinkage of the crust consequent on cooling.

Can we point out some analogies to this on the Earth?
Certainly.

The defile of the Jordan, terminating in the awful depression of the Dead Sea, no doubt occurs to you on the moment.