of the lowland trunk streams the great arch
was developed so slowly that these channels could be maintained through
_pari passu_ deepening. Former tributaries have given place to others
developed with reference to the distribution of more or less easily
eroded strata, the present longitudinal valleys being determined by the
out-crop of soft shales or soluble limestones, and the parallel ridges
upheld by hard sandstones or schists. Parallelism of mountain ridges and
intervening valleys is thus attributable to the folding of the rocks,
but the origin of the interior structure of the mountains is to be kept
distinct from the origin of the mountains as features of topography.

_Flora and Fauna._--Much of the region is covered with forest yielding
quantities of valuable timber, especially in Canada and northern New
England. The most valuable trees for lumber are spruce, white pine,
hemlock, cedar, white birch, ash, maple and basswood; all excepting pine
and hemlock and poplar in addition are ground into wood pulp for the
manufacture of paper. In the central and southern parts of the belt oak
and hickory constitute valuable hard woods, and certain varieties of the
former furnish quantities of tan bark. The tulip tree produces a good
clear lumber known as white wood or poplar, and is also a source of
pulp. In the south both white and yellow pine abounds. Many flowering
and fruit-bearing shrubs of the heath family add to the beauty of the
mountainous districts, rhododendron and kalmia often forming
impenetrable thickets. Bears, mountain lions (pumas), wild cats (lynx)
and wolves haunt the more remote fastnesses of the mountains; foxes
abound; deer are found in many districts and moose in the north.

_Influence on History._--For a century the Appalachians were a barrier
to the westward expansion of the English colonies; the continuity of the
system, the bewildering multiplicity of its succeeding ridges, the
tortuous courses and roughness of its transverse passes, a heavy forest
and dense undergrowth all conspired to hold the settlers on the
seaward-sloping plateaus and coastal plains. Only by way of the Hudson
and Mohawk valleys, and round about the southern termination of the
system were there easy routes to the interior of the country, and these
were long closed by hostile aborigines and jealous French or Spanish
colonists. In eastern Pennsylvania the Great Valley was accessible by
reason of a broad gateway between the end of