50 NEW YORK STATE MUSEUM 
Accompanying the Taconic disturbance and possibly aiding the 
metamorphism were minor molten rock intrusions in the form of 
dikes. These dikes break through late Ordovicic strata and hence 
can not be older than late Ordovicic. A fine example of one of 
these dikes on Manhattan island is shown in plate 24. 
The great compressive force which folded and upraised the 
Taconic mountains did not accomplish its work suddenly. The 
force was slowly and irresistibly applied and the mass of strata 
was gradually bulged and bent, or fractured if near the surface, the 
amount of ‘time required for the whole operation being perhaps 
very long but beyond estimate. Such a length of time as, however, 
so short compared with all known geologic history that we are 
accustomed to refer to the formation of such a mountain range as 
simply an event of earth history. 
From these statements we see that, even before the range had 
attained its maximum height above sea level, a very considerable 
amount of erosion must have taken place. When the very first 
fold appeared above the ocean level, erosion began its work and 
continued with increasing vigor as the mountain masses got higher 
and higher. Thus we have the warfare between two great natural 
processes — the building up and the tearing down. So long as the 
building up process predominated, the mountain range increased in 
elevation, and we say the range was in its period of youth. When 
the opposing forces were about equally balanced, the range tended 
to remain at a constant elevation and we say the mountains were 
in the period of maturity. When the tearing down (erosive) 
process was predominant, we speak of the range as having been in 
old age. When the mountains have been completely worn down 
close to sea level (peneplain) we speak of the death of the range. 
Here is an example of one of the remarkable procedures of 
nature. After millions of years of work by the deposition of 
thousands of feet of strata, layer upon layer on an ocean bottom, 
a great compressive force is brought to bear and a magnificent 
mountain range is literally born out of the ocean. No sooner is 
this great mountain range well formed than the destructive 
processes unceasingly destroy this marvelous work. But the sedi- 
ments derived from the wear of this range are carried into the 
nearest ocean again to accumulate and after long ages to be raised 
up into another range; and so the process is often repeated. From 
this we learn that the mountain ranges of the earth are by no 
means all of the same age. The Adirondacks are older than the 
Taconics, and these older than the Appalachians; the latter, in turn, 
