The streaks, in many cases, would not be visible at all were it not for their different colour, which is considerably lighter than that of the surroundings. The only explanation offered for this fact is the assumption that the original cracks were filled by some light matter forced out from the interior of the Moon, that is by the lunar magma. This magma was not very viscous, as it has spread out considerably beyond the edges of the cracks proper. These presumably, like those on Earth, were of a rather moderate width, not enough to be distinguishable at the Moon’s distance. Similar light-flowing emanations from long fissures are known also on our planet, for instance, from the Laki eruption on Iceland in 1783. The colour may be light simply by comparison with the previously solidified crust, which, optically, as regards reflexion of light, has proved very similar to obsidian or, even more like another volcanic mineral product, vitrophyre. It is also possible, however, that gas bubbles were liberated as the lava solidified and gave the surface a milkwhite appearance--gravity on the Moon is only one sixth of that on the Earth so that the bubbles would rise and evaporate extremely slowly from the magma. Due to the very low atmospheric pressure on the Moon, the bubbles would also occupy a larger volume than in a corresponding case on Earth and become more conspicuous in proportion. They probably partly remained on the surface of the outpoured lava as a thin scum, which hardened in that state. Since then, it has suffered no more change than all other formations on the Moon, while on the Earth it would soon have been scoured away by sand and water. [Illustration: Fig. 29. The great lunar crater Copernicus surrounded with streaks. Below the Carpathian mountain range and at bottom part of Mare Imbrium. The moon diameter corresponds to 55 cm. Photo by Yerkes Observatory. ]