SURFACE MARKINGS.
Mr. James Croll, in a letter to "Nature" (July 13, 1876), incidentally mentions the lessons that may be derived from the configurations of the earth's surface.
"Given the hardly perceptible wearing of water and time, a cañon a mile deep, and many hundreds of miles long, has resulted from the flowing of a stream. Given glacial 'abrasion' and time enough, then valleys of rounded section and firths and lake-basins of a particular kind probably resulted from the flowing of ice.
"Where a stream flows from source to mouth on a gradual slope, there has been no great disturbance of level since the stream began to work. Where ice fills the dales there are no cañons. Where ice has filled dales and has left fresh marks, cañons are short and small. In mountain regions, where ice-marks are rare or absent, cañons are of great depth and length, apparently because their streams have flowed in the same channels ever since the mountains were raised. But where cañons are marked features, these lakes, firths, and dales of rounded section are very rare, or do not exist. It seems therefore that hollows which have, in fact, been carved out of the earth's surface may be known for water-work or for ice-work by their shape, and that firths, dales, and lakes may mark the sites of local glacial periods; and cañons the sites of climates that have not been glacial since the streams began to flow."