The Holyoke trap sheet begins south of Amherst and extends 80 miles across Massachusetts and Connecticut to Long Island sound. I t is about 300 feet thick in the latitude of Holyoke and has a low dip eastward, resting conformably in the Triassic sandstones (see plate 24), and plainly was a submarine flow.* It and its neighbors are composed for the most part of a monotonous diabase; but peculiar structures have been locally produced by the introduction in various ways of the mud, sand, and water of the sea bottom into the interior of the liquid and moving mass.
I have elsewhere described considerable areas where the trap, protected below by a thin solid crust, has flowed over and rested on . . .