310 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
The chief rocks of this district are metamorphic. ‘The dominant 
structures, belts of gneiss, schist, and limestone, strike N. N. E. 
After the surface of these rocks had been maturely dissected, it was 
glaciated and, along the western part, depressed below sea-level, 
and partly buried by marine and fresh-water deposits which are 
largely of glacial origin. In many of the valleys further east there 
are large bodies of water-laid glacial materials. Changes of local 
water-levels during the retreat of the ice-front, and successive eleva- 
| tions of the region (of which the area under consideration is a part) | 
| have lead to a considerable dissection of the deposits just mentioned. | 
| In describing in more detail those parts of Vermont and Quebec 
with which we are concerned, it is convenient to divide the area on a 
topographic basis into an eastern highland, which includes part of the 
Green Mountain Highland; and a western lowland, which includes 
the southern part of what may be called the Champlain Lowland 
(Plate 2, fig. A). | 
| Tue Hicuianp.— The highest parts of the main ridge of the | 
| Green Mountains are in the highland part of the area. Several | 
points on the ridge are over 4,000 feet in altitude. Mt. Mansfield, 
which occupies a central position in the area studied, rises 4,364 feet 
above the sea. Through this ridge and lower ridges on the east and 
west, the three largest west-flowing rivers of Vermont have cut deep 
water-gaps. The depth to which these gorges were cut before they 
were partly filled with glacial materials is not precisely known, for the 
glacial materials have not yet been sufficiently removed or explored. 
South of Mt. Mansfield where the Winooski River has cut through 
the ridge the gravelly stream bed is less than 330 feet above the sea. 
North of this mountain the bed of the Lamoille River is scarcely 150 
feet higher. 
Several of the longitudinal subsequent valleys east of the main | 
ridge are but little higher than the water-gaps through which they | 
are drained. For example, the valley next east of Mt. Mansfield, 
the northern part of which drains to the Lamoille River, and the 
southern part to the Winooski River, is only 740 feet in altitude at the 
divide on the floor of the valley, between these streams. 
The northern ends of some of the longitudinal valleys northeast 
of the one just described drain northward from similar low divides 
into Lake Memphremagog, thence into the St. Lawrence River; some 
of the valleys southeast, drain southward through branches of the 
l White River, into the Connecticut River. 
| Tur Lowrand.— The lowland part of the area studied is bounded 
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