MUD CRACKS ON SHORE OF ROGER'S LAKE, ARIZONA

In order to pronounce the lines on Mars as simply cracks one should study the various kinds of cracks in similar surfaces on the Earth. In such a study he would be amazed at the similarity of cracks. When there is a grain in the substance, as in wood, the cracks follow the grain, though even in this material they are discontinuous. In amorphous material they have essentially the same character; whether in the almost microscopic crackle of old Satsuma pottery, or huge cracks in sun-dried mud, the areas enclosed are generally polygonal. If the material be of impalpable fineness the edges of the cracks are smooth and clean-cut, as in Plate III, from a Chinese bowl; whereas if the material is coarse and pebbly the edges of the cracks are rough and irregular, as in Plate IV, from the muddy shores of a lake. Cracks arising from contraction never converge to a common centre, and when not connected with another crack they taper to a point. They begin at indefinite places and end in an equally indefinite manner. That there should be a common resemblance in cracks due to contraction is evident as they arise from a shrinking of the surface. The most ancient deposits, millions of ages ago, reveal mud cracks differing in no respect from those found to-day. We subjoin a few forms of cracks from various surfaces, to show their essential resemblance. It will be seen that the cracks in the Moon are identical in character to those found on the Mesa at Flagstaff. They start from some indefinite point, are irregular in outline and end as indefinitely. A poor asphalt pavement offers one of the best opportunities for the study of the formation of various kinds of cracks and fissures. On the edge of a sloping sidewalk one may see the cracks due to a sliding, or lateral displacement of the surface; the effects of subsidence show a number of cracks around the area of depression; the growth of a tree crowding the asphalt shows the effect of lateral thrust, and an enlargement of a root below, or the effects of frost show cracks due to elevation. All these various cracks reveal the same features: they are discontinuous, they begin and end without definition. Schiaparelli says in regard to the canali of Mars: "None of them have yet been seen cut off in the middle of the continent, remaining without beginning or without end." These lines on the surface of Mars, as a writer in "Nature" says, are almost without exception geodetically straight, supernaturally so, and this in spite of their leading in every possible direction. It is inconceivable that cracks should be laid out with such geodetic precision. We have seen that cracks have no definite beginning or termination; we have seen that the lines of Mars begin and end at definite places. Cracks are irregular, vary in width and differ entirely from the straight lines depicted by Schiaparelli, Lowell, and others. But if we admit them to be natural cracks in the crust we are compelled to admit that the forces implicated in such cracks must have been active many millions of years ago, as Mars, being a much older planet than the Earth, must have long since ceased to show those activities which the Earth, even to-day, exhibits in such phenomena as earthquakes, subsidences, elevations, and the like. Now cracks made at that early time in the history of the planet must have long since become filled with detritus and obliterated in other ways, and no evidence would show, even on close inspection, of their former existence, much less at a distance of 50,000,000 of miles, more or less.

PLATE V

1. POTTERY CRACKLE
2 INCHES		2. MUD CRACKS
2 FEET
3. ASPHALT CRACKS
16 INCHES		4. EARTH CRACKS
10 FEET
5. CRATER CRACKS, MOON
55 MILES		6. a. MOON b. AFRICA
100 MILES 1500 MILES

NATURAL LINES
CRACKS, FISSURES, ETC.

In Plate V, page 112, are given six figures representing various cracks and fissures. No. 1 represents the cracks in the glaze of Japanese pottery, magnified. No. 2 shows the mud cracks on the edge of a lake, to the extent of two feet. No. 3 is a series of cracks in an asphalt pavement, covering about two feet. No. 4 shows the form of cracks in the surface of a mesa in Arizona, the result of the summer heat, the length being about ten feet. No. 5 is a tracing from a drawing by Professor W. H. Pickering showing cracks in the lunar crater Eratosthenes, with an extent of fifty-five miles. The original drawing represented a much greater widening of the lines which Professor Pickering believes to be due to vegetation. I endeavored to trace the centre of each line and Professor Pickering said in regard to my tracing: "In one or two instances you have assumed that a crack went through the middle of a broad space, whereas, for aught we know, it may have gone along either edge, but otherwise the tracing obviously follows the outlines of my drawing." It evidently gives a cachet of what appears to be veritable cracks on the surface, and it is interesting to compare this drawing with the cracks in the asphalt. In No. 6 are two drawings; one marked A represents cracks in a region of the Moon known as Flammarion's Circle, the other B represents the great rift in southern Africa, probably the most stupendous phenomenon in geological history. This rift has been traced from the Valley of the Jordan through the Dead Sea, into the Gulf of Akaba, thence into the Red Sea, which it follows the entire length, then turning southwesterly into Africa and branching, one branch takes in Lake Tanganyika, and the other branch Lake Nyassa. A portion north of Nyassa is still problematical. Here is a crack 1,500 miles long, most of it filled with detritus, water, or forest. It would be an interesting question whether such a fracture would be visible even from the Moon. A glance at these various figures will give one a conception of the similarity of cracks, their irregular contour, their indeterminate origin, and ending. Cracks arising from shrinkage vary only in the material in which the crack takes place; the conditions resulting from shrinkage or pulling apart are precisely the same.

PLATE VI