A second feature in the river system of Haute Loire is that a certain number of the affluents of the Loire run into it from the direction in which it is flowing, and their mouths are more or less against the stream they are about to feed. A river usually affects the form of a deciduous tree, of which the branches represent the tributaries. The branches are attached to the trunk at an obtuse angle, as seen from below. With pines it is different; with them the limbs are attached in the reverse fashion, at an acute angle as seen from below. Some of the affluents of the Loire come into it in the way in which a fir branch grows out of the main trunk. This is notably the case with the Arzon and the Borne. The reason for this is that the basin of Le Velay has a rim to the north, and the drainage from the north naturally runs down to the lowest point in the basin. But on reaching this spot the streams come on the Loire, which has cut for itself a huge gap through the northern lip of the bowl, and is able to discharge its waters through that. How this was done demands some explanation. The Rhône is the mightiest of the rivers of France, but its sources are in the Swiss Alps, and it does not enter France till after it has passed through the Lake of Geneva. But the Loire is next in size and importance, and it flows through French soil only. The source is under the Gerbier de Jonc, in the department of Ardèche, but flows in it for a short course only. It is still a feeble stream when it enters the department of Haute Loire, through which it makes its way till it leaves it for Forez. The Loire rises 4,500 feet above the sea, and when it quits the department it has fallen to 1,450 feet. When the volcanoes of Le Velay were in eruption and the tableland was overflowed with molten lava, the Loire must have been arrested and have mounted to heaven in a column of steam. In time the lava cooled, and the stream groped for beds of scoria and fallen dust through which it could nibble its way with ease. But when it encountered a barrier of basalt the case was altered. This blocked its course as with rows of iron piles rammed into the ground and running far back. But the crystallisation of the lava into basaltic prisms helped the river to break through. The molten matter, of the consistency of treacle, flowing over the country followed its undulations, filling hollows here and rounding obstructions there. When it cooled it began to crystallise, and form hexagonal columns that are upright. But when the surface of the original soil would not allow of regular crystallisation, there the columns shaped themselves in all directions and in great confusion; the result being that in many places the basalt was fractured, fissured, and ruinous from the very first. The water speedily detected these weak points, worked at them, tumbled the columns down, overleaped them, bored further, and did not rest till it had cut its way completely through the barrier.