56 A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 



of the sphere, either to form a coating on the surface about a vulcanoid, or 

 to stain a belt of rocks on either side of the rift, so that a strip of country, 

 perhaps a mile or more in width, extending to the top of the crust, was impreg- 

 nated with the material — the deposit being perhaps more extensively accumulated 

 on the surface. 



As already noted, there is reason to suspect that, besides the large reflecting 

 power which the materials of the largest rays possess when the sun is high, these 

 materials have a certain fluorescent property, which causes them in some measure 

 to store up light which is given out after the sun has passed the angle at which they 

 begin to shine. That such is the case is indicated by the fact that the rays are visi- 

 ble on the limb of the moon when the sun's light is considerably more oblique than 

 it is when they become very bright. Such a property is known to exist in many 

 species of minerals. A close study of fluorescence may, indeed, serve to indicate 

 the nature of the substance which sends us the light from the very shiny parts 

 of our satellite, — that of the diffused patches as well as that of the rays. 



If we accept the hypothesis that the bright parts of the moon are due 

 to the deposition of some highly reflecting and perhaps fluorescent materials, 

 we may proceed to derive certain important corollaries from the proposition. 

 It is at first sight evident, from the extent of the shining fields on and about the 

 ramparts of the greater vulcanoids, that the egress of the light-reflecting materials 

 was there by so many paths that the resulting stains were confluent, and that the 

 rays marked its passage in fields where the channels were rarer, though related 

 to the same centers of vulcanoid action. It is also evident that, while these pas- 

 sages for vapors from within cut a few of the crater floors of lava and occasionally 

 extend on to the maria, they appear never to originate in those areas. More- 

 over, the great extent of these rays, some of them exceeding one thousand miles 

 in length, and the way in which they radiate from their several centers, are prob- 

 lems of no small importance. 



As to the common origin of the blotches of light material on and about the 

 vulcanoids and the rays, the series of facts leaves no good reason for doubt. 

 The blotches generally pass outwardly by gradations into rays, the most of which 

 are short, perhaps less than a score attaining a length of one thousand miles or 

 more. As to the deep-seated origin of these structures, it is fairly proved by 

 the fact that they cross irregularities of the surface, as well as by the fact that 

 they occur along lines of craterlets. There is some reason for believing that 

 these, the smallest of the vulcanoids, were formed along the lines of the rills, 

 presumably before those clefts were opened. Their existence along the light 

 rays is of itself evidence that there is some incipient breakage along their 

 courses. It is a reasonable supposition that vapors were forcing themselves out 

 on those lines, and that sometimes they did so with explosive energy. 



The existence of incipient crevices, such as jointings arranged in a general 

 radial order with reference to the greater vulcanoid centers and extending for 

 very great distances, is a feature which from the point of view of the geologist is 

 surprising. While in the case of terrestrial volcanoes it is common to find traces 



