If it revolves about a vertical axis d its centre of gravity must always lie in that axis; if it rolls the centre of gravity must always lie over the e abutment.
This involucre is frequently composed of several rows of leaflets, which are either of the same or of different forms and lengths, and often lie over each other in an imbricated manner.
Patches of white pumice often lie over the black sand, and when it is thick enough it is easily rideable.
Make sure any fasteners are well-covered or designed to lie over a strip of fabric so that they won't rub on your skin.
