The deep placers of California are often covered by a thick capping of lava; in other places, the eruptive matter overlying the auriferous gravels occurs in the form of columnar basalt, beneath which is found the auriferous strata of sand, clay, and gravel. These deposits often contain immense numbers of large tree-trunks, which, with those portions of their branches which remain, are either silicified, or are converted into a lignite, often containing a considerable amount of iron pyrites.