Yet we are not ourselves by any means devoid of 'straight lines' structurally produced, in spite of every obstacle of diversity of form and texture, of softness and hardness, of lamination or crystallisation, which are adverse to such developments. Examples of these are the numerous 'faults' which occur in the harder rocks, and which often extend for great distances in almost perfect straight lines. In our own country we have the Tyneside and Craven faults in the North of England, which are 30 miles long and often 20 yards wide; but even more striking 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._