Our illustration, Plate XIX., is a photograph from one of many glass globes which we have cracked in the manner described: a careful comparison between the arrangement of the divergent cracks represented in the photograph and those seen spreading from Tycho and other lunar craters will, we trust, justify us in what we have stated as to the similarity of the causes which have produced such identical results.
The accompanying figures will further illustrate our views upon the causative origin of the bright streaks. The primary action rent the solid crust of the moon and produced a system of radiating fissures (Fig. 42): these immediately afforded egress for the molten matter beneath to make its appearance on the surface simultaneously along the entire course of every crack, and irrespective of all surface inequalities or irregularities whatever (Fig. 43). We conceive that the upflowing matter spread in both directions sideways and in this manner produced streaks of very much greater width than the cracks or fissures up through which it made its way to the surface.
[Illustration: Fig. 42. ILLUSTRATIVE OF THE RADIATING CRACKS WHICH PRECEDE THE FORMATION OF THE BRIGHT STREAKS.]
In further elucidation of this part of our subject we may refer to a familiar but as we conceive cogent illustration of an analogous action in the behaviour of water beneath the ice of a frozen pond, which, on being fractured by some concentrated pressure, or by a blow, is well known to “star” into radiating or diverging cracks, up through which the water immediately issues, making its appearance on the surface of the ice simultaneously along the entire course of every crack, and on reaching the surface, spreading on both sides to a width much exceeding that of the crack itself.
[Illustration: Fig. 43. ILLUSTRATIVE OF THE RADIATING BRIGHT STREAKS.]
If this familiar illustration be duly considered, we doubt not it will be found to throw considerable light on the nature of those actions which have resulted in the bright streaks on the moon’s surface. Some have attempted to explain the cause of these bright streaks by assigning them to streams of lava, issuing from the crater at the centre of their divergence and flowing over the surface, but we consider such an explanation totally untenable, as any idea of lava, be it ever so fluid at its first issue from its source, flowing in streams of nearly equal width, through courses several hundred miles long, up hills, over mountains, and across plains, appears to us beyond all rational probability.