incavation

“When I threw these pieces into the lava, they occasioned that dull kind of sound which would have been produced by striking soft earth or thick mud; and at the same time formed in the lava an incavation, in w'lich they were buried about one third part of their bulk, and in this situation were carried away by the current.”

Internet Archive: A General collection of the best and most interesting voyages and travels in all parts of the world [microform] : many of which are now first translated into English : digested on a new plan