Sentence:  An excellent guidebook by Drs Kilburn and McGuire of University College London reveals that these unpromising pieces of debris are <WSD>scoria</WSD> and lithic fragments of the March 1944 eruption. 
Meaning of the word marked with <WSD> tags:  Rough masses of rock formed by solidified lava, and which can be found around a volcano's crater. (from 18th c.)