228 



THE POPULAR SCIENCE MONTHLY. 



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 Trias- 

 sic formation, ly- 

 ing 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 consid- 

 ered 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, aggre- 

 gating ten or twelve thousand feet in thickness at least, but al- 

 ways 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 accumu- 

 lation of the bedded rocks there were at least three epochs of con- 

 siderable 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 hun- 

 dred feet. These correspond to the bed marked A in Fig. 8, 

 which represents a magnified view of a corner of the block seen 



Fig. 7. 



