[Illustration: a. _Columnar lava rests upon red sandstone in the
cliffs at Greenfield._]
[Illustration: b. _Fissures were filled with liquid rock that became
solid and bonded wall to wall at the Windsor Dam._]
Every one of these masses flowed into the rocks along fractures and
other zones of weakness, crystallizing as they lost their heat and
solvents to the hot springs of that ancient time. They are all invaders,
or intrusives, which inserted themselves into the older beds. Whether
they were squeezed into the fissures by the pressures that crumpled the
original beds into their upturned positions, or whether they, like the
liquid in a hydraulic press, transferred pressure from a deep reservoir
to the walls of the fissures and so pushed the beds into their distorted
forms, is unknown. Two features are clear; the distortion of the beds
and intrusion of the liquid bodies were almost simultaneous, and the hot
springs associated with them were still active at the dawn of Triassic
time. These profound disturbances transformed the land into a series of
elevated, wave-like folds, and rains promptly began to tear away at the
summits of the newly raised mountains. From them was carved a serrate
and rugged landscape, part of which was later buried beneath the
Triassic fill.
_Coal Swamps in Massachusetts and Rhode Island_
Among the strata of the Eastern Upland which were folded, intruded,
baked hard, and stewed in hot spring water, one group stands
pre-eminent. It forms a broad band starting north of Worcester and
reaching to Providence and beyond. Nearly everywhere it carries coaly
material or impressions of plants which are now extinct, but which
flourished in the Coal Age or Carboniferous period. Some of the coal
seams were mined in the Providence basin, but they had been so heated by
intrusive granite that they are partly graphitic and proved difficult to
burn. The great extent of some of the coal seams suggests a panorama of
immense swamps, and of land so flat that, for long periods, streams
brought no sediment, and the trees and water-loving plants furnished the
only fill. At other times sluggish rivers, flowing from the northwest,
laid thick layers of sandy mud over the surface of the bogs. The
alternating muds and coal seams are thousands of feet thick, and they
record the story of a basin which sank as fast as it filled--a
depression which was never built high enough to be a well drained plain,
yet never subsided sufficiently to be inundated by the sea. The
Carboniferous peat bogs and mud flats may have extended westward almost
to the Connecticut Valley; and farther to the northwest they were
bounded by a chain of rolling hills.
The rock floor of the coal basin contains a variety of ancient
materials. Some rocks were river deposits, some were marine limestones,
a few were lava and volcanic ash, and many were granite and gneiss which
crystallized at great depths and became exposed only after streams had
stripped away the thick overburden. The basin floor thus holds a complex
story, in which land and sea, vulcanism and quiet, erosion and
deposition, all played their respective roles. Only in the west, along
the margin of the Connecticut Valley, is the involved story at all
clear. And in the Western Upland across the red-rock inlay, it is
possible to see some of the land as it was before trees took root in the
swamps, and rivers brought sands and muds from the vegetated hills that
hemmed in the coal basin.
_The Western Upland_
Many years ago, when transportation facilities were not what they are
now, New England settlers mined iron ore from the hill north of
Bernardston and smelted it in local charcoal furnaces. The rocks
containing the iron are creased into sharp, close folds, and they came
into such close contact with a hot granite intrusive that their minerals
were changed by its action. This granite, however, is older than the one
which is associated with the disturbed Carboniferous beds, for it was
intruded when the Devonian sediments from Gaspé to Connecticut were
deformed. It was this profound disturbance that turned the red rocks of
Roche Percé from a horizontal to a vertical position and raised a
mountain range which stretched through all of northern New Brunswick,
Maine, the lowland section of New Hampshire, and a belt extending for
some miles east and west of the Connecticut Valley. The eroded remnants
of these Shickshock Mountains formed the backdrop for the great
Carboniferous coal swamps in Rhode Island, Massachusetts, and Acadia.
The iron ore was a hot spring replacement of a limestone containing
shells of sea organisms which lived when chordate animals first became
abundant. This was the Devonian period in geologic history--the time
when a backbone appeared essential in every really high-grade animal.
The limestone rests upon a beach gravel, now consolidated into a
quartzite conglomerate. The gravel consisted of small white quartz
pebbles which came from the many veins in the steeply inclined slates of
the adjacent coast.
Marine deposits of Devonian age are found as far south as Leverett, and
scattered outcrops indicate that the old seaway reached northward up the
Connecticut, entering Canada east of Lake Memphremagog. Thence it spread
eastward to Gaspé and westward to Montreal, and around the north and
west side of the Adirondack Mountains into New York State. A low rolling
land where the Green Mountains stand today formed the western shore of
the Devonian sea for many miles northward into Quebec. The Adirondack
and Taconic Mountains were a fused aggregate of undulating uplands which
limited the seaway on the south along the International Boundary. Its
eastern shore lay far beyond the horizon of the region described in this
brief account.
The rocks of the old Devonian coast in Massachusetts were chiefly
slates, cut by many quartz veins. They are exposed along the Mohawk
Trail in the ascent from Greenfield to Shelburne Summit, and they
continue northward in an almost unbroken band through Bernardston,
Brattleboro, and Northfield (Vermont) to Lake Memphremagog. They contain
casts of planktonic life which inhabited the Ordovician seas in these
northern latitudes, and the Ordovician strata, together with still older
Cambrian sediments found below them, meet the Devonian beach deposits at
a sharp angle, just as the slates along the coast of Maine meet the
modern beach sands and gravels. Like the slates of Maine, they were
eroded deeply before the beach existed, and their slaty structure and
their steeply inclined attitudes were acquired in a still more ancient
epoch of deformation.
The folded rocks of Ordovician age flanked the highland area which now
constitutes the axis of the Green Mountains. West of the Green Mountains
they make the Taconic Range, and to the east they appear in ranges that
go under a variety of names, including the Northfield and the Lowell
Mountains. In the Taconics the folds have the shape of waves advancing
westward from the center of disturbance in the Green Mountain axis;
within the Connecticut basin the Ordovician folds have wave-fronts which
advance from the same axis eastward across the Memphremagog sea. Along
the eastern margin of the old land a series of dark green intrusives
called peridotite welled up from the depths of the earth, and they now
cut through the rocks extending from Chester, Massachusetts, to Thetford
Mines, Quebec; they are like giant boundary posts marking the ancient
line of demarcation between sea and land in Cambro-Ordovician time.
Originally the folded strata in the Taconic region were deposited in
clear marine waters, where calcium carbonate accumulated rapidly. But
the sediments of the same age east of the Green Mountain land represent
an unbroken succession of hardened muds, which rest on sandy muds, and
on fine and coarse products of violent volcanic eruptions--tuffs and
agglomerates--and lava flows. No lime-secreting animals could thrive in
this sea, although they numbered billions in the western waters; for
only floating plankton could escape the interminable mud, and they
drifted up and down the coast from Quebec to Connecticut. One or two
straits may have connected the clear waters of the west with the muddy
waters of the east, for some of the planktonic organisms have been found
in the muddier sediments of the westerly waterbody.
The Cambro-Ordovician sea lapped even older rocks, contorted and cut by
intrusives which bonded them precisely as much younger invading liquid
rock bonded the younger sediments of the Eastern and Western Uplands.
The older rocks were also laid in a sea--a sea so much more ancient than
the Cambrian and Ordovician seaways that its shoreline and even its form
and extent are at best conjectural. And when we study these oldest
marine beds, we find that their ingredients were in part derived from
still more ancient sedimentary rocks, which accumulated in the sea, and
that these old beds were elevated into the land that supplied the waste
now found in the oldest coherent section of rocks in western New
England. Indeed, the dawn of the Cambrian period, when life first became
abundant, was merely a half-way mark through geologic time. Although
half a billion years have elapsed from the Cambrian to the present,
another half a billion years reach still farther back towards the
beginnings of earth history, beyond which science has not yet peered
successfully. These billion years are but a finite segment of history,
bounded by the infinite past and the infinity of the future.
It seems appropriate, therefore, to end our journey down the fourth
dimension at this point, and as we retrace our steps, we can profitably
survey the chronologic succession of events and scenes which followed
each other from Cambrian time to the Twentieth Century A.D.