On April 19, 1903, when, after being hidden for thirty days, owing to the different rotation periods of the two planets, the Mare Erythraeum, the largest blue-green region of the disk and lying in the southern hemisphere, rounded again into view, a startling transformation stood revealed in it. Instead of showing blue-green as usual, and as it had done six weeks before, it was now of a distinct chocolate-brown. It had been well seen at its previous presentation, so that no doubt existed of its then tint. At that time the Martian season corresponded to December 30 in our calendar. Eighteen Martian days had since elapsed, and it was now January 16 there. The metamorphosis had taken place, therefore, shortly after the winter solstice of that part of the planet. The color change that had supervened proved permanent. For the next night the region showed the same brown hue, and so it continued to appear throughout the days that it was visible. Two months passed, and then the chocolate hue had vanished,—gone as it had come,—and the _mare_ had resumed its usual tint, except for being somewhat pale at the south. It had come to be February 21 on Mars. Timed and tabulated, the metamorphosis through which the _mare_ passed stands out thus:— MARE ERYTHRAEUM 1903 ============+================+=============+================== | DAYS BEFORE OR | | | AFTER SUMMER | | MUNDANE | SOLSTICE | MARTIAN | DATE | (BEFORE = - | DATE | ASPECT | AFTER = +) | | ------------+----------------+-------------+------------------ February 16 | -10 | December 16 | Blue-green March 20 | +22 | January 1 | Blue-green April 19 | 52 | January 16 | Chocolate April 22 | 55 | January 18 | Chocolate May 26 | 89 | February 4 | Faint chocolate May 30 | 93 | February 6 | Faint chocolate June 30 | 123 | February 22 | Faint blue-green July 7 | 130 | February 25 | Faint blue-green ============+================+=============+================== The culmination of the transformation seems to have taken place about 60 days after the southern winter solstice, or in the depth of the Martian winter of that hemisphere. This is certainly just the time at which vegetation should be at its deadest. The northern and southern portions of the _mare_ did not behave alike in taking on the chocolate tint. From the notes made about them during the opposition it appears that the latter was later than the former in undergoing the metamorphosis, as will be seen from the following depth of the blue green estimated in percentages shown at different dates, calling the deepest tone ever exhibited by it unity.