Dura Den, in addition to the interest arising from lithological structure, presents an excellent example of a valley of erosion. The river which traverses it rises at times into considerable volume, and sweeps with violence through the pass; connected above, at one period, with a lake, and acting continuously on soft friable matter, the abrading powers of the instrument are sufficiently adequate to the production of the effect. The qualities of the rocks penetrated may be easily inferred from the windings of the stream—the harder substances occasioning a divergence from the straight course—the soft and marly scooped out into wider and more extended areas. A section of any one of them is thereby labeled for the fullest inspection, which are arranged, not perpendicularly one upon another, but drawn out in longitudinal succession on the floor and sidewalls of the valley, and exhibiting to the geologist, after so many types and forms of the old red sandstone, the first break and most northern limit of the coal metals in the great central basin of Scotland. THE YELLOW SANDSTONE, as it is termed from its prevailing color, though not uniformly so, belongs to the old red or devonian system of rocks, of which the cornstone and conglomerate beds are in the immediate vicinity, and the position and relation of the three to one another easily determinable. The upper or yellow deposit occupies the valley of Stratheden nearly throughout its entire length and breadth, and ranges along the base of the heights of Nydie, Cults, the Lomonds, Binnarty, and the Cleish hills, dipping under the carboniferous lower group, and generally separated by overlying masses of trap. The sandstones, indeed, of both systems, resemble each other so much in color and texture, that in many instances along the line now indicated the trap must be taken as a guide by which to ascertain the qualities and respective positions of the two series. Glenvale, a beautiful ravine which intersects the Lomond range, presents admirable sections of the whole group, in their regular order of superposition and finely displaying their contrasting mineral characters.