is the great Cleveland Dyke—­a wall of volcanic rock dipping slightly towards the south, but sometimes being almost vertical, and stretching across the country, over hill and dale, in an almost perfect straight line from a point on the coast ten miles north of Scarborough, in a west-by-north direction, passing about two miles south of Stockton and terminating about six miles north-by-east of Barnard Castle, a distance of very nearly 60 miles.  The great fault between the Highlands and Lowlands of Scotland extends across the country from Stonehaven to near Helensburgh, a distance of 120 miles; and there are very many more of less importance.

Much more extensive are some of the great continental dislocations, often forming valleys of considerable width and length.  The Upper Rhine flows in one of these great valleys of subsidence for about 180 miles, from Mulhausen to Frankfort, in a generally straight line, though modified by denudation.  Vaster still is the valley of the Jordan through the Sea of Galilee to the Dead Sea, continued by the Wady Arabah to the Gulf of Akaba, believed to form one vast geological depression or fracture extending in a straight line for 400 miles.

Thousands of such faults, dykes, or depressions exist in every part of the world, all believed to be due to the gradual shrinking of the heated interior to which the solid crust has to accommodate itself, and they are especially interesting and instructive for our present purpose as showing the tendency of such fractures of solid rock-material to extend to great lengths in straight lines, notwithstanding the extreme irregularity both in the surface contour as well as in the internal structures of the varied deposits and formations through which they pass.

Probable Origin of the Surface-features of Mars.

Returning now to Mars, let us consider the probable course of events from the point at which we left it.  The heat produced by impact and condensation would be likely to release gases which had been in combination with some of the solid matter, or perhaps been itself in a solid state due to intense cold, and these, escaping outwards to the surface, would produce on a small scale a certain amount of upheaval and volcanic disturbance; and as an outer crust rapidly formed, a number of vents might remain as craters or craterlets in a moderate state of activity.  Owing to the comparatively small force of gravity, the outer crust would become scoriaceous and more or less permeated by the gases, which would continue to escape through it, and this would facilitate the cooling of the whole of the heated outer crust, and allow it to become rather densely compacted.  When the greater portion of the gases had thus escaped to the outer surface and assisted to form a scanty atmosphere, such as now exists, there would be no more internal disturbance and the cooling of the heated outer coating would steadily progress, resulting at last in a slightly heated, and later in a cold layer of moderate thickness and great general uniformity.  Owing to the absence of rain and rivers, denudation such as we experience would be unknown, though the superficial scoriaceous crust might be partially broken up by expansion and contraction, and suffer a certain amount of atmospheric erosion.