The Government engineers engaged upon the ship-canal around the rapids where the Columbia river cuts through the Cascade Mountains, and the engineers of the Oregon Railway and Navigation Company, whose railroad runs beside the Government Canal, have discovered that a point of the mountains, of tremendous height and three miles in extent, is moving down an incline into the river. The fact of a moving mountain is strange, but not incomprehensible. It seems, says an intelligent correspondent of the New York Times, that the great river and the ravines that point to it have cut their way down through a superincumbent mass of basalt into a substratum of sandstone. This sandstone, we will suppose, presents a smooth surface, with an incline towards the river; the river cuts under the basalt into the sandstone, and the natural effect is for the superincumbent basalt, acting like a similar formation of ice in a glacier, to slide down hill. It furnishes an illustration of the manner in which some of the more remarkable changes in the earth's surface have been produced, and will doubtless lead to further scientific investigation.