A Thing of the Past — John Russell Fearn

Something is wrong in England. In and around the London area, people are suddenly becoming bad-tempered, with a rapid increase in crime and murder, and a clear decline in self-respect. Linked to this strange behavior is an excavation site, now a mighty smoking crater emitting clouds of noxious gas that appear to be causing the sudden change in human behavior. And then, from within the vast fissure below the crater, there emerge the hideous survivals of a lost age of monster dinosaurs. It's up to Cliff Brooks, project engineer, to develop a mighty boring capsule capable of penetrating deep within the earth, where he hopes to close off the source of the gas. But when an accident disables his machine a thousand miles from any source of help, only Herbert the Dinosaur can save day! A rollicking good SF adventure tale, the first in a two-book series.