The “bluestone” which is found in the central parts of some of the concretions associated with the large spiral bodies described by Mr. B. B. Woodward is found, on microscopical examination, to be a calcareous sandstone. Sabangular grains of quartz, averaging 0·04 mm. in diameter, constitute over 50 per cent of the rock, and are set in a cement of carbonates which have crystallized in plates very much as in the Fontainebleau sandstone. These plates are sometimes 3 cm. across, and give a characteristic lustre to the stone. The quartz-grains are evenly distributed through the plates of crystalline carbonate, and are so closely packed as to be almost in contact. It is easy to imagine that with the complete removal of the carbonates by solution there would remain a friable sandstone, very porous, but still occupying almost the same volume as the bluestone.