BIRT.—­This ring-plain, about 12 miles in diameter, is situated on the Mare Nubium, some distance due E. of Thebit.  It has a brilliant border, surmounted by peaks rising more than 2000 feet above the Mare, and a very depressed floor, which does not appear to contain any visible detail.  A bright crater adjoins it on the S.W., the wall of which at the point of junction is clearly very low, so that under oblique light the two interiors appear to communicate by a narrow pass or neck filled with shadow.  I have frequently seen a break in the N.W. wall of Birt, which seems to indicate the presence of a crater.  There is a noteworthy cleft on the E., which can be traced from the foot of the E. wall to the hills on the N.E.  It is a fine telescopic object, and, under some conditions, the wider portion of it resembles a railway cutting traversing rising ground, seen from above.  It is visible as a white line under a high light.

THE STRAIGHT WALL.—­Sometimes called “the railroad,” is a remarkable and almost unique formation on the W. side of Birt, extending for about 65 miles from N.E. to S.W. in a nearly straight line, terminating on the south at a very peculiar mountain group, the shape of which has been compared to a stag’s horn, but which perhaps more closely resembles a sword-handle,—­the wall representing the blade.  When examined under suitable conditions, the latter is seen to be slightly curved, the S. half bending to the west, and the remainder the opposite way.  The formation is not a ridge, but is clearly due to a sudden change in the level of the surface, and thus has the outward characteristics of a “fault” Along the upper edge of this gigantic cliff (which, though measures differ, cannot be anywhere much less than 500 feet high) I have seen at different times many small craterlets and mounds.  Near its N. end is a large crater, and on the W. is a row of hillocks, running at right angles to the cliff.  No observer should fail to examine the wall under a setting sun when the nearly perpendicular E. face of the cliff is brilliantly illuminated.

NICOLLET.—­A conspicuous little ring-plain on the E. of Birt, and somewhat smaller.  Between the two is a still smaller crater, from near which runs a low mountain range, nearly parallel to the straight wall, to the region S.E. of the Stag’s Horn Mountains.  Here will be found three small light-surrounded craters arranged in a triangle, with a somewhat larger crater in the middle.

PURBACH.—­An immense enclosure of irregular shape, approximating to that of a rhomboid with slightly curved sides.  It is fully 60 miles across, and the walls in places exceed 8000 feet in altitude, and include many depressions, large and small.  On the E. inner slope are some fine terraces and several craters.  The continuity of the circumvallation is broken on the N. by a great ring-plain, on the floor of which I have seen a prominent cleft and a crater near the S. side.  There is a large bright crater in the interior of Purbach, S. of the centre, two others on the W. half of the floor, and a few ridges.