Some of them were as straight as if laid out with a line, others were curved a little here and there, though still maintaining the strict parallelism of their sides.

These crossed each other; those entered craters and came out at the other side.

Here, they furrowed annular plateaus, such as _Posidonius_ or _Petavius_.

There, they wrinkled whole seas, for instance, _Mare Serenitatis_.
These curious peculiarities of the lunar surface had interested the astronomic mind to a very high degree at their first discovery, and have proved to be very perplexing problems ever since.

The first observers do not seem to have noticed them.