In the shallower tropical waters, especially on the central ridge, considerable areas are covered by Pteropod ooze, a deposit consisting largely of the shells of pelagic molluscs.
All interruptions are not so costly, for in shallower waters, with favourable conditions of weather, a repair may be only a matter of a few hours, and it is in such waters that the majority of breaks occur, but still a large reserve fund must be laid aside for this purpose.
In depths beyond the reach of wave motion, and apart from suspension across a submarine gully, which will sooner or later result in a rupture of the cable, the most frequent cause of interruption is seismic or other shifting of the ocean bed, while in shallower waters and near the shore the dragging of anchors or 40 fishing trawls has been mostly responsible.
It is deeper and more sandy where granite is the underlying rock, deeper and more fertile on the north-western than on the south-eastern mountain slopes, and shallower and more clayey where slate is the underlying rock.
The most effective tool against the weeds is a broad sharp " sweep," as it is called, which takes everything it meets, while going shallower than most ploughs.