40 NEW YORK STATE MUSEUM 
(see plate 17 and figure 12). That these dike rocks were intruded 
after the great pressure and uplift of the region is shown by the 
total absence of metamorphism or alteration of any kind along their 
contact lines. The fine-grained texture of these rocks, often with 
borders of glass, shows that they must have cooled close to the sur- 
face, and hence it is evident that most of the Precambric erosion of 
the region had been accomplished before the diabases were erupted. 
Such rocks suggest that there may have been volcanic activity at the 
surface but no positive proof for such activity can be given because, 
if such volcanic material ever existed, every trace of it has been re- 
moved by erosion. The diabases have been observed to cut the 
pegmatites and hence they are not only the younger of the two, but 
they must take rank as the youngest Precambric rocks in the State. 
In the Adirondacks the pegmatites are very widely distributed and 
common, while the diabases are most abundant in the northeast, 
less so in the northwest and southeast, and nearly absent from the 
southwest. : 
