In chapters one and five, Golding describes the platform. It is depicted in chapter one as:
a great platform of pink granite thrust up uncompromisingly through forest and terrace and sand and lagoon to make a raised jetty four feet high. The top of this was covered with a thin layer of soil and coarse grass and shaded with young palm trees. There was not enough soil for them to grow to any height and when they reached perhaps twenty feet they fell and dried, forming a criss-cross pattern of trunks, very convenient to sit on. The palms that still stood made a green roof, covered on the underside with a quivering tangle of reflections from the lagoon.
In chapter five, the platform's uncomfortable aspects are noted. This is symbolically important because the discomforts—the sacrifices—of being part of civilization, which the assemblies on the platform represent, are increasingly less alluring than the indulgent life of atavistic pleasure offered by Jack:
This palm trunk lay parallel to the beach, so that when Ralph sat he faced the island but to the boys was a darkish figure against the shimmer of the lagoon. The two sides of the triangle of which the log was base were less evenly defined. On the right was a log polished by restless seats along the top, but not so large as the chief's and not so comfortable. On the left were four small logs, one of them—the farthest—lamentably springy.
The mountain top is also described in chapter one, when the boys explore the island and get their bearings:
They were on the lip of a circular hollow in the side of the mountain. This was filled with a blue flower, a rock plant of some sort, and the overflow hung down the vent and spilled lavishly among the canopy of the forest. The air was thick with butterflies, lifting, fluttering, settling.
We learn, too, that it has a square top.
The mountain, like the platform, has important symbolic significance. As the description above shows, it is a place of great, almost Edenic beauty, symbolizing its divine aspect. It is the vantage point from which the boys can see most clearly. From this high point, they can take in the lay of the whole island and the water surrounding it: in fact, it is not until they arrive here that they can even be sure they are on an island. It is also the logical place to build a fire to try to attract rescue. Rightly used, it can become a place where a positive spirituality--a bright hope for the future—can merge with the rationality of tending to a rescue fire. Unfortunately, however, spirituality can go awry and mountain top experiences can devolve into experiences of fearful superstition that drive out reason, as happens to the boys.