ROCK MATERIAL. 23 
fiver Lake, about three-fourths of a mile long. (See Pis. XIII 
Id XIV, pp. 46, 48.) 
TOPOGRAPHIC TYPES. 
Three well-defined local topographic types stand out from a survey 
m the Taconic landscape. 
I Plateau type. — This includes the Green Mountain Range, with its 
Ipntly undulating or roundish surfaces, having few and usually not 
jigged elevations, its flank deeply incised; also the Rensselaer 1*1 a - 
|au, which is a miniature westerly inclined peneplain, about 175 
luare miles in area, with very few and slight elevations and some 
load, shallow valleys. Its edge is slightly incised. 
I Taconic type. — This consists of alternating ridges and valleys. 
[he ridges are cut by transverse and diagonal valleys and are some- 
Imes intricately dissected or else reduced to isolated lenticular masses. 
1 Tludson-Champlain type. — This is less marked in character, con- 
Isting of minor, irregular elevations and depressions, and yet with a 
■pries of isolated hills reaching to nearly TOO feet above its general 
■irface. 
ROCK MATERIAL. 
This description and this analysis of the land forms are here 
|atu rally followed by a description of the rock material which con- 
Ititutes or underlies these forms. The map, PI. I, is not properly a 
leological map, for it does not show the geological formations, but 
lie distribution of the rock areas with reference to their principal 
flhemical and physical characteristics without reference to age, and 
lb may therefore be termed a rock map. 
The map differentiates the following areas: Those underlain by 
Bocks, mainly granular and crystalline (quartzite, metamorphic con- 
glomerate, metamorphic grit or graywacke, and gneiss) ; those under- 
lain by rocks mainly schistose (sericite-quartz-chlorite-schist) ; those 
jimdexlain by the less metamorphic slate or by unaltered shale and 
trit; and those underlain by calcareous rocks (limestone, dolomite, 
lind marble). It was not practicable to separate slate from shale 
jior to outline the very small areas of limestone or quartzite or grit 
occurring within the schist, slate, and shale. The granular rocks 
lilso include some interbedded schist and slate. The western boundary 
pf the schist marks approximately the transition from greater meta- 
Imorphism on the east to less on the west. The area (on the map 
j|Without rock pattern) west of that boundary and east of the Hudson 
and Lake Champlain consists almost entirely of slates, shales, and 
^rits, the slates lying usually east of the shales, although in some 
pases their areas are interlaced. The area of undesignated material 
along the eastern edge of the map on the Green Mountain Range is 
