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THE GEOLOGICAL HISTORY OF NEW YORK STATE 39 
that many thousands of feet of materials have been removed by 
erosion in order to expose the present rocks to view does not neces- 
sarily imply that the mountains at any time had so great a height be- 
cause it is possible that, while elevation slowly progressed, material 
was steadily removed by the process of erosion. All our knowledge 
regarding later and better known mountains, however, leaves little 
doubt that those first Adirondacks were mountaims vastly higher 
than those of today. 
The same sort of uplift and succeeding profound erosion also 
affected the present region of the Hudson Highlands (see figure 
14) and, this being the case, we can state with confidence that 
much, if not all, of northern and eastern New York was in- 
volved in this mountain-making process. For western New York 
the physical geography of this time has not been determined because 
all that region is so deeply buried under the Paleozoic strata. . 
LATE PRECAMBRIC HISTORY 
The fact that such a great thickness of rock was removed by ero- 
sion. implies a vast length of time for the accomplishment of that 
work. According to all that we know regarding the rates of erosion 
of mountains of the present and past, the erosion of the Adiron- 
dacks must have extended over a period of at least several mil- 
lion years. When we consider that, at the opening of Upper Cam- 
bric-time, most of the region had been worn down to the condition 
of a peneplain (see page 42), we are confident that no small 
amount of the erosion was accomplished even before the opening 
of the Paleozoic era, and that it continued well into the early Paleo- 
zoic. The whole problem of this later erosion and its effects will be 
treated in the next chapter. 
Well toward the end of Precambric time, igneous activity of a 
minor character took place in the formation of dikes which, as we 
have learned, are fissures in the crust of the earth which have been 
filled with molten rock. In the Adirondack region there are sev- 
eral kinds of dike rocks, the most common being pegmatite and dia- 
base. Pegmatite is a very coarse-grained, light colored rock of gen- 
eral granitic composition, the feldspar and quartz crystals often at- 
taining lengths of several inches to a foot. Diabase is a fine-grained 
rock much like ordinary basaltic lava. These dikes are generally 
less than a mile long and comparatively narrow. That they are 
younger than the other igneous rocks of the region is abundantly 
proved by the fact that they cut through those rocks in many places 
