248 W. N. THAVER 
with the mainland only through the eastern axis, and he states that 
the western axis ends in an arc at the end of the Gaspé Peninsula. 
The physiographic history of the New England region begins 
with the close folding that occurred near the end of the Pennsyl- 
vanian epoch. But although this folding controls the topography, 
the detailed forms so far as known are the net result of two periods 
of base-leveling and subsequent uplift, one in Jurassic-Cretaceous 
and the other in Tertiary time, modified by glaciation. 
Of the history of the Canadian Appalachians, Goldthwait* 
says: 
During the closing part of the Mesozoic subaérial denudation seems to 
have held sway. The mountains were slowly but surely reduced to a plain 
of low relief or “‘peneplain.” Locally, in districts remote from the coast and 
where stronger rock structure appeared just above the Cretaceous base-level, 
the reduction of the surface was incomplete and many residual mountains or 
monadnocks were left.. On the whole, however, the base-leveling was very . 
thorough, planing away the harder rocks as well as the weaker. 
This almost complete cycle of denudation was brought to a close at about 
the beginning of the Tertiary by regional uplift. The uplift seems everywhere 
to have been greatest in the interior and least near the coast. By it the 
seaward-flowing rivers were revived and a new cycle of erosion was begun, 
and by mid-Tertiary time broad lowlands had been developed. Another up- 
lift occurred and the lowlands were carved by the streams until a fairly ma- 
ture topography had been evolved beneath the Tertiary surface. 
THE LAURENTIAN PLATEAU 
Lying with its vertex near the Great Lakes a U-shaped area of 
pre-Cambrian rocks stretches away to the north, inclosing Hudson 
Bay in its arms, and extends into the still unexplored regions of the 
Arctic. It embraces nearly all of the northeastern part of North 
America and projects two spurs, the Adirondack Mountains and 
the Superior Highlands, into the United States. 
The western and southern boundaries of this division have been 
described or have had their locations implied in previous para- 
graphs. The location of the northern boundary cannot be stated 
with accuracy, but may be placed tentatively near the sixty-eighth 
parallel. The northeastern boundary of the plateau proper is a 
group of mountains that lie close to the coast and extend from the 
«J. W. Goldthwait, Geol. Survey Canada, Guide Book, No. 1, Part I, pp. 6-7. 
