Page:Popular Science Monthly Volume 40.djvu/240

 slopes of the volcanic island, for leaves of cycads are found in the neighboring beds of shales. And yet all this is gone. The volcanoes are only things of the imagination. The Blue Hills mark the conduits through which they were fed with lavas, but the cones are lost in the empty air above; only the deep roots of the structure are now preserved for us.

Perhaps the accompanying diagrams may aid the reader in gaining a fuller understanding of the geological history of the region. They are drawn from a wooden model that was prepared for exhibition before the Geological Society of America at its last winter meeting in Washington. The first (Fig. 7) represents a block of the Triassic formation, lying horizontally on its deep crystalline foundation, the whole representing a cube of about ten miles on a side, and hence showing a hundred square miles of upper sur—face. The oblique lines across the top need not be considered for the present. The horizontal lines around the sides near the top are the interbedded lava-sheets, and all these, with the sandstones and shales, lie on the upturned eroded edges of the foundation of old crystalline rocks. The bedded rocks were spread out in the old sinking estuary in deposits of great volume, aggregating ten or twelve thousand feet in thickness at least, but always in shallow water, for they frequently show cross-bedding and ripple marks, and sometimes mud-cracks and rain-drops, and occasionally even foot-prints of various kinds. The famous Hitchcock collection, in the Amherst College Museum, illustrates all these features in great variety. During the period of accumulation of the bedded rocks there were at least three epochs of considerable volcanic activity. About half of the total thickness of the strata had been deposited when the first outburst took place, and this is the one that yielded the ashes and bombs at Meriden. Its lava-flows spread many miles north and south, but gained only a moderate measure of thickness, generally not more than a hundred feet. These correspond to the bed marked A in Fig. 8, which represents a magnified view of a corner of the block seen