"They don't seem to mind the sulphur and the mud," said Fenn to himself. The lad had read in his school books of the mud volcanoes. They are of a type similar to the hot geysers of Yellowstone Park, though not so large or numerous. Though called boiling springs in some parts of the country they do not boil or bubble on the surface, as a rule, though there is a constant supply of warm water from some subterranean source, so, that, as in the case with the spring Fenn was viewing, the water ran over from the pool, and trickled off through the woods.