818 TriA.\SACTioxs of the American Institute. 



tinetly the character of the rajs which proceed from it. The four 

 largest form mountain ranges of coubiderable nuignitude; one of 

 which runs directly away from the observer ; and though distinctly 

 elevated at its point of emanation, far above the general level of the 

 moon's surface, it gradualh' lowers until, before disappearing over the 

 opposite horizon, it has spread itself like a delta amongst tlie ordinary 

 debris of the surface ; another, which takes a direction nearly at right 

 angles to this, forms a grand mountain range of nearly 500 

 miles in length, decreasing in altitude, but widening as it recedes 

 from its source, until it finally breaks like a great stream ai^ainst the 

 rampart of Gauss, a ring formation 110 miles in diameter ; here it is 

 lost and dispersed, like its companion, over the common level; a third 

 and fourth take still different directions, and have all the characteris- 

 tics of their fellows, though not of equal magnitude, while in the 

 angles formed by those ridges other rays emanate in mass, as it were, 

 without separating into ridges, submerging the surrounding country 

 as they proceed until all traces of their destructive character is lost 

 in the ordinary surface level, leaving a very gradual descending grade 

 from their center of emanation outward, while transversely, or 

 across the rays the surface would be comparatively level. The 

 greater part of the grand ray system which surrounds Tycho, is of 

 this character, though careful observation will show a difference of 

 level between the brighter streaks and the portions in lower tints, but 

 as a general thing they are very badly situated for observations in this 

 respect, as the rays of the rising and setting sun are nearly parallel 

 to the finest groups, and therefore unfavorably situated for develop- 

 ing the delicate nuirkings of light and shade which a surface so nearly 

 smooth will give under the most favorable circumstances. But there 

 is one of those streaks whicli radiates from this basin, so different in 

 appearance from the rest of the group as to make it evident at a 

 glance that it must have had a history peculiar to itself, and that 

 local causes have operated at an early period of its formation to pro- 

 duce the distinctive difference of character which it presents. 



It is well known that upon our earth, rivers which form delta like 

 the Nihi or the Mississippi, build their own banks above the general 

 level of the deposit ; and if we could imagine about ten miles of the 

 Mississippi abandoned by its waters, we would have a tolerably fair 

 miniature representation of this mighty lunar chasm of somewhat 

 over 300 miles long. The massive banks of this structure 

 rise far above the surrounding country, overtopping even the ram- 



