GEOLOGY OF THE VICINITY OF LITTLE FALLS i 



ress. As the surface was worn away, the present surface rocks 

 were brought that amount nearer to the surface and were under 

 the load of a constantly diminishing amount of overlying rock. 

 From time to time renewed side pressures were brought to bear, 

 these likely coinciding with times of renewed uplift and disturb- 

 ance. But, as the rocks approached the surface in this slow 

 fashion, the effects produced on them by the pressure would 

 change -in character. The pressure seems also to have been less 

 pronounced than in the former stage and to have been mainly ef- 

 fective in producing joints.^ Likely slipping and faulting also 

 took place, but if so the faults have not yet been differentiated 

 from those of a much later period. 



Toward the close of this long erosion period, after much the 

 larger part of the overlying rocks had been worn away, volcanic 

 activity was renewed in the Adirondack region. The main center 

 of the activity of this period was in the northeastern Adirondacks, 

 and igneous rocks of this period make but little show in the south. 

 They used mainly an eastwest set of joint planes for their ascent. 

 We do not know whether any of this material reached the surface 

 or not. If so, all traces of the surface materials have been since 

 worn away. Nor has erosion anywhere cut deeply enough to dis- 

 close the old reservoirs whence these lavas arose, though they were 

 the same in all probability as those whence came the material for 

 the earlier great intrusions. We find exposed at the surface of 

 today merely the old, lava-filled fissures which served as the chan- 

 nels of ascent for the molten rock. The surface outflows have 

 disappeared through erosion and the original reservoirs are still 

 buried in depth. 



These rocks are known to be distinctly later than the other 

 igneous rocks by various sorts of evidence. The dikes (the filled 

 fissures) of the later rocks cut through the earlier. The later 

 rocks are not metamorphosed as are the earlier. They have suf- 



^Joints are divisional planes, usually vertical or highly inclined, found 

 in most rocks. There are in general at least two set's of planes nearly at 

 right angles. There may be several sets. They result from both tension 

 and from compression, and are only formed comparatively near the 

 surface. 



