REPORT OF THE DIRECTOR AND STATE GEOLOGIST 1*45 



Daring or shortly after this deformation period occurred the 

 last outbreak of igneous activity in the vicinity, molten rock again 

 rising from below through the set of east and west fissures. If 

 any of this material ever reached the surface, all trace of it has* 

 been worn away, and we see at the present time only the dikes, 

 as the old fissures are called through which the rock worked its 

 way upward and in which it cooled as the activity ceased, filling 

 them solidly from side to side. The Adirondack region was not 

 a center of the igneous activity of this period but on the border, 

 the dikes of this age occurring along the shores of Lake Cham- 

 plain but not extending back from it for any considerable dis- 

 tance. No dikes of this period have been discovered within the 

 limits of the Mooers sheet, though they are found along the lake 

 shore in the territory covered by the Plattsburg sheet, next south- 

 east. As in the preceding time of igneous rock formation, there 

 were at least two separate periods of activity, the rock of the one 

 being a light colored, acid rock, and of the other a black, basic 

 rock. There are constant differences between these rocks and the 

 preceding diabase and syenite, such that where both occur they 

 may be distinguished. The acid rock followed, instead of preced- 

 ing the basic, as in the earlier period. With this outburst igneous 

 activity in the region terminated. 



There is a possibility that during late Upper Silurian or early 

 Devonian time the region was again depressed below sea level for 

 a short time, at least that the Champlain valley was. The valley 

 came into being as a topographic feature with the faulting of 

 the previous deformation period. There is evidence that the St 

 Lawrence valley at Montreal was below sea level at the time 

 specified, and it is simply inferred that the Champlain valley may 

 have been also affected. If so, every vestige of the deposit laid 

 down on that sea floor has since been eroded away. With the ex- 

 ception of this possible short time of depression the district was 

 a land area throughout the entire vast interval from the close of 

 Lower Silurian time to the comparatively recent departure of the 

 Pleistocene ice sheet. This interval certainly represents at least 

 three fourths of the entire elapsed time since the beginning of de- 



