GEOLOGY OP THE NORTHERN ADIRONDACK REGION 347 



time only in dikes. We see the cluannels through which the 

 material ascended, but can not be sure whether any reached the 

 land surface of the time, giving rise to true volcanic action, nor 

 do we anywhere get a glimpse of the underlying reservoirs which 

 supplied the material, since erosion has nowhere cut deeply 

 enough to disclose them. It may well be, therefore, that the 

 mere dikes give little idea of the possible importance of this 

 period of igneous activity. But, if great surface flows occurred, 

 or volcanos were formed, it seems strange that no vestiges of 

 their presence remain, since, as has been stated, the character 

 of the dikes themselves does not indicate any very gTeat amount 

 of erosion of the present surface as compared with that of that 

 time. The most of the erosion since has been expended on the 

 paleozoic cover which subsequently overspread this old land 

 surface. 



These dikes apparently owe their existence to the same causes 

 which were responsible for the earlier, great intrusions, and mark 

 the last paroxysm of igneous activity from that source. They 

 are wholly unmetamorphosed and are the only Precambric rocks 

 in the region of which this is true. Moreover, in cooling, the 

 chilling influence of the walls has been very marked, indicating 

 comparatively near surface conditions at the time. The borders 

 of even the largest dikes have cooled so rapidly as to be glassy, 

 though the rock may become quite coarsely crystalline toward 

 the center. Little or no trace of such strong, chilling effect is 

 to be found in the older dikes of the region. Occasionally the 

 dikes are even somewhat amygdaloidal, which is also indicative 

 of cooling at no great depth. These characters point conclusively 

 to a much younger age for these dikes. Their time of appearance 

 was not only subsequent to the great metamorphism of the region, 

 but was near the close of the following long period of Pre- 

 cambric erosion. Before their appearance the rocks which they 

 cut had had the greater part of the overlying load of material 

 which covered them during metamorphism, laboriously pared 

 away by the slow processes of erosion, a depth of erosion being 

 involved which necessarily argues the lapse of a vast interval of 

 time. 



These rocks are very similar to those of the igneous outflows 

 which characterized Keweenawan time (supposedly late Pre- 



