PICKERING. LUNAR AND HAWAIIAN PHYSICAL FKATURES COMPAItKI). 173 
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scale. If so, the other side must have been destroyed by a subsequent melt in 
leaving an unusually smooth, light-colored surface in its place. 
It was later concluded that these valleys were produced by a continuous faulting 
along a line of volcanic weakness, and were therefore analogous to the craters, where 
instead the faulting extends in all directions from a volcanic centre. The best 
Hawaiian representative of the ( se grooved valleys is therefore probably the great 
crater of Haleakala (Figure 3), whose length measures seven miles and its breadth 
two. These are about the relative proportions found in many of the valleys south- 
west of Pallas, — nor are their dimensions so very different. The line of six small 
craters found in the bottom of Haleakala corresponds to the similar line of small 
craters found along the minute rill in the bottom of the Valley of the Alps. 
The fact that often one and sometimes both ends of the lunar valleys are closed 
by high walls, as is the case with Haleakala, strengthens the second explanation of 
opposed 
Elongated craters for 
step between the ordinary craters and the grooved valleys are of frequent occurrence 
upon the Moon. The largest of these is Schiller, in the southeastern quadrant. A 
nameless one is shown in the lower left-hand corner of Figure 31, and another in the 
lower right-hand corner of Figure 29. Others are shown on the border of the mare 
in the same figure. 
The lunar rills may be divided into two classes, — rills and crater rills. The rills 
proper are extremely numerous upon the Moon. About a thousand are already 
known. The Ariadseus rill, shown in Figure 29, is the widest and most conspicuous 
of them. It measures three miles in breadth by a little over half a mile in depth, as 
determined by the shadows of the ridges that cross it in various places. Like all true 
rills its course is approximately straight, or made up of curves of long radius. In its 
bottom are several minute craterlets not shown in the photograph. Evidently like 
our dikes and mineral veins it has been partially filled from below. Other narrower 
rills, apparently bottomless, are found on the Moon. Two much smaller parallel rills 
with a north and south direction are found upon the mure to the left. One of these 
is faintly shown in the photograph. The general view that the rills are simply 
cracks in the lunar surface is undoubtedly correct. They occur most frequently 
in formations of the secondary period, that is in the dark surfaces, or if found in the 
primary formations, it is where the surface has apparently been softened and par- 
tially flattened out by the application of heat, as in the present instance. Rills 
are frequently found at the edges of the mark and running parallel to them, as in 
Serenitatis and Humorum. 
