Chapter 7

They had struck a cave. Early in the boring the sandstone had given place to a vein of Comanchean limestone, full of minute fossil cephalopods, corals, echini, and spirifera, and with occasional suggestions of siliceous sponges and marine vertebrate bones--the latter probably of teliosts, sharks, and ganoids. This, in itself, was important enough, as affording the first vertebrate fossils the expedition had yet secured; but when shortly afterward the drill head dropped through the stratum into apparent vacancy, a w…