When Desmarest argued for the volcanic origin of basalt, he was working with rocks that were often columnar in form and could be almost directly connected with extinct craters. But there were other kinds of rock formations, which we now know had a volcanic origin, that were more difficult to understand. The most notable of these were the "toadstones" of Derbyshire, which lie buried deep underground and have no columnar structure at all.
The illustration shows a detail of a section near Derby (for the full section, see exhibit item 44). The three black layers filled with white air holes are the toadstones.