Similar in behavior to it is the long chain of so-called islands that, beginning southwest of Thaumasia, runs thence westward even to the eastern edge of Hellas. These belt the planet in a west-northwesterly direction by a strip of territory from ten to fifteen degrees wide, the medial line of which begins at 55° south and ends in about 40°. They are parted from the main bright areas by blue-green ‘seas’ of about the same width as themselves, the Mare Sirenum, the Mare Cimmerium and Mare Tyrrhenum. These ‘seas’ the white that covers the ‘islands’ never crosses; though the continent, as we may call it for convenience, descends at the east to 30° south. Since the ‘seas’ are not seas, the cause which might bound the snow, were they such, cannot be the cause here. Nevertheless, they have an effect of some sort on the isothermal lines as is shown not only by latitudinal comparison with the state of things in Hellas, but with that in Thaumasia as well. For 30° south is also the limit apparently of the white on Thaumasia, where ochre desert stretches ten degrees farther south still; the region in its southern part being white-mantled, in its northern part not. Here again, then, the ochre areas make exception to what affects the blue-green ones. Clearly the blue-green regions temper the action of what gives them wintry cloak. But why they should do this is not easy to explain on any supposition terrestrial or marine. Bodies of water tend to foster the formation of clouds; so, less markedly, do areas of vegetation. Neither the old ideas, then, nor the new lend themselves in explanation. It may be that while we here seem to be envisaging cloud we are in reality looking at hoar-frost. On the other hand, light cloud would show less, superposed over a dark background, than over an ochre one; and this, the simplest of all explanations, may be the true one. It is facts like these that intrigue us in the study of the Martian surface by revealing conditions which render offhand analogy with the earth unsafe. Indeed, we are more sure of some things which appear too strange to be true than of others so simple on their face as to enlist belief. Among the most difficult and perplexing are meteorological problems like the above. Here we can only say provisionally that while cloud best answers to the appearance, frost best fits the cause. For vegetation might melt frost, yet not dissipate cloud. By raising our conception of the mean temperature the facts can, however, be reconciled and this is probably the solution of the difficulty after all.