GEOLOGY OP FRANKLIN COUNTY 123 



Eruptive rocks of Silurian age. Igneous activity did not cease 

 in the Adirondack region with the close of the period of diabase 

 extrusion, but with a much later period of dike formation. The 

 eruptive center of this period seems to have been in New England, 

 and the dikes found in New York are practically limited to the 

 immediate shores of Lake Champlain. Kemp has described sev- 

 eral from Essex county, and the writer a few from Clinton. Both 

 acid and basic rocks occur, but in each case they differ somewhat 

 from the Precambrian eruptives in type, so that there should be 

 no difficulty, usually, in making determinations of age. No dikes 

 which can be ascribed to this period have been found in Franklin 

 county, and the strong probability is that they do not occur. 



PALEOZOIC ROCKS 



Much later than all the previously described rocks of Franklin 

 county are the Potsdam sandstone, of Upper Cambrian age, and 

 the Calciferous dolomites and limestones, of Lower Silurian age, 

 which are found in the northern part of the county only. The 

 overlying Chazy and Trenton limestones do not appear in the 

 county so far as known. These rocks have been briefly discussed 

 in a previous report, and no additional work has been done on 

 them. 1 



These rocks were laid down on a much eroded floor of the older 

 rocks. Apparently the Adirondack region was a land area for a 

 larger part, if not the whole, of the long time interval between the 

 deposition of the Grenville series and of the Potsdam sandstone. 

 During this interval the amount of material eroded away from the 

 region was very great, apparently greater than the entire amount 

 removed since Silurian times, since which it has also been con- 

 tinuously above sea level. 



A single observation made far within the area occupied by the 

 Precambrian rocks, in 1897, is of considerable interest. An old 

 limekiln stands near the road running north from Alderbrook 

 postoffice to the north branch of the Saranac, somewhat over 2 

 miles from the former. Before visiting the place it was inferred 



iCusliing, H. P. 16th an. rep't N. Y. state geologist. 1896. (geol. map) 

 p. 5-27. 



