Chapter 22

One, though spacious and inviting, opened on a seemingly bottomless abyss without visible means of descent. Now and then we had a chance to study the petrified wood of a surviving shutter, and were impressed by the fabulous antiquity implied in the still discernible grain. These things had come from Mesozoic gymnosperms and conifers--especially Cretaceous cycads--and from fan palms and early angiosperms of plainly Tertiary date. Nothing definitely later than the Pliocene could be discovered. In the placing of these…