the southern basins, as about Richmond, Virginia, are valuable
beds of coal; what was the physical geography of these areas when
the coal was being formed?

Interbedded with the Triassic sandstones are contemporaneous lava
beds which were fed from dikes. Volcanic action, which had been
remarkably absent in eastern North America during Paleozoic times,
was well-marked in connection with the warping now in progress.
Thick intrusive sheets have also been driven in among the strata,
as, for example, the sheet of the Palisades of the Hudson,
described on page 269.

The present condition of the Triassic sandstones of the
Connecticut valley is seen in Figure 315. Were the beds laid in
their present attitude? What was the nature of the deformation
which they have suffered? When did the intrusion of lava sheets
take place relative to the deformation? What effect have these
sheets on the present topography, and why? Assuming that the
Triassic deformation went on more rapidly than denudation, what
was its effect on the topography of the time? Are there any of its
results remaining in the topography of to-day? Do the Triassic
areas now stand higher or lower than the surrounding country, and
why? How do the Triassic sandstones and shales compare in hardness
with the igneous and metamorphic rocks about them? The Jurassic
strata are wanting over the Triassic areas and over all of eastern
North America. Was this region land or sea, an area of erosion or
sedimentation, during the Jurassic period? In New Jersey,
Pennsylvania, and farther southwest the lowest strata of the next
period, the Cretaceous, rest on the eroded edges of the earlier
rocks. The surface on which they lie is worn so even that we must