1. A Martian came across the fields about midday, laying the stuff with a jet of superheated steam that hissed against the walls.

划线部分是定语从句。that 作从句主语。

2. The country northward was as though a black snowstorm had passed over it.

划线部分是表语从句从句。as if/though...用在 be 动词后，是表语从句。