"Here's the mystery," I cried as he rushed up to us, "and a deadlier scheme of Dame Nature's to frighten and murder poor humanity I have never seen."
As I spoke I lit the lamp again and held it just above a tiny fissure in the rock. It was at once extinguished.
"What is it?" said Bainbridge, panting with excitement.
"Something that nearly finished me," I replied. "Why, this is a natural escape of choke damp. Carbonic acid gas—the deadliest gas imaginable, because it gives no warning of its presence, and it has no smell. It must have collected here during the hours of the night when no train was passing, and gradually rising put out the signal light. The constant rushing of the trains through the cutting all day would temporarily disperse it."
As I made this explanation Bainbridge stood like one electrified, while a curious expression of mingled relief and horror swept over Henderson's face.
"An escape of carbonic acid gas is not an uncommon phenomenon in volcanic districts," I continued, "as I take this to be; but it is odd what should have started it. It has sometimes been known to follow earthquake shocks, when there is a profound disturbance of the deep strata."
"It is strange that you should have said that," said Bainbridge, when he could find his voice.
"What do you mean?"
"Why, that about the earthquake. Don't you remember, Henderson," he added, turning to the Inspector, "we had felt a slight shock all over South Wales about three weeks back?"
"Then that, I think, explains it," I said. "It is evident that Pritchard really did climb the rocks in a frantic attempt to escape from the gas and fell back on to these boulders. The other man was cut down at once, before he had time to fly."