Tahiti is a lofty green island, with deep folds of a darker green, in which you divine silent valleys; there is mystery in their sombre depths, down which murmur and plash cool streams, and you feel that in those umbrageous places life from immemorial times has been led according to immemorial ways. Even here is something sad and terrible. But the impression is fleeting, and serves only to give a greater acuteness to the enjoyment of the moment.

请教老师：

在定语从句down which murmur and plash cool streams中，这里的 cool 是主语补语吗？