Page:Popular Science Monthly Volume 66.djvu/153

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Fossil footprints have been found in various parts of the world, as in England, Germany, France, and, in our own country, in the Grand Canon of the Colorado, as well as in association with Jurassic dinosaurs in the northwest; but it is in the valley of the Connecticut and in rocks of the same formation in New Jersey that they occur in an abundance and perfection of preservation which is unrivaled elsewhere.

In this region sandstone beds of great thickness occur which every now and then exhibit impressions with generally something of an interval between the track-bearing layers.

Geologists have been led to suppose that the broad valley of the Connecticut was during Triassic times a tidal estuary extending from the village of Northfield, Mass., to New Haven, a distance of one hundred and ten miles, with an average width of twenty miles. In places in this estuary were mud flats, some well out in the ancient bay, others nearer the shore, which were left bare by the receding tides. Here the animals loved to congregate, possibly they came for food, but it seems more likely that the dinosaurs here assembled at certain seasons for mating as the seals do in the Alaskan rookeries.

The means of preservation were threefold, of which the first was the fierce heat of a tropical sun, for plant remains indicate that such climatic conditions prevailed, while the second and third are really attributable to one cause, volcanic activity. This resulted in the formation of the Holyoke and Mt. Tom ranges and in broad lava flows. Upon these sheets the sand and silt were deposited, forming thus the tidal flats where the creatures congregated. The heat of the cooling lava added its baking effect to that of the sun, while the decomposing lava liberated an iron cement which completed the task of solidifying the overlying material into rock.

The impressions made when the tide had ebbed were thus somewhat hardened before the incoming flood bearing its burden of sediment gently buried the traces without the least injury, thus preserving for our enlightenment these monuments of the past.