问一个so的用法

制约与平衡

这是OG第一题的正确的句子。
1.The Glass House Mountains in Queensland, Australia, were sighted in 1770 by the English navigator CaptainJames Cook, who so named them supposedly because their sheer wet rocks glistened like glass.
请问各位who so中的那个so是什么词性？是翻译成什么吗？

制约与平衡

求求大家告诉我吧……