ADIRONDACK MAGNETIC IRON ORES 19 



and crushing have been carried to an extreme, the resultant gneisses 

 present most difficult problems to the geologist since they are 

 often inextricably involved with the sedimentary gneisses. 



The age of the granite intrusions relath^e to that of the other 

 igneous rocks has been demonstrated in only a few cases. There 

 is little question that some granites are later than the anortho- 

 site and according to Professor Cushing even later than the syen- 

 ite. Not improbably they may represent more than one period of 

 intrusion. 



Dike rocks. The dike rocks in the Adirondack region are mostly 

 diabases. These are common in Clinton county, where they have 

 been uncovered in large numbers in the mines, and to a lesser 

 extent in Essex county. In the interior and on the southern and 

 western sides they occur only rarely. The dikes seldom attain a 

 thickness of more than 20 or 30 feet, the majority perhaps being 

 less than 10 feet. A few dikes of syenite porphyry have been found 

 in Essex and Clinton counties. The dikes cut all the formations 

 previously described, but have not been found anywhere to inter- 

 sect the Paleozoic strata. They belong thus to the Precambric. 

 That they must have been intruded very late in Precambric time 

 is indicated by the fact that they have undergone no appreciable 

 metamorphism or compression in which the other crystalline rocks 

 have participated. 



Paleozoic sediments. The Paleozoic sedimentary strata, which 

 are found on the edges and to a lesser extent in the interior of the 

 Adirondacks, rest in nearly horizontal position upon the eroded 

 surface of the crystallines. During the period of their deposition 

 the region underwent a gradual subsidence that brought a contin- 

 ually increasing area, with the progress of time, below the level of 

 the sea. The formations have at their base the Potsdam sand- 

 stone, while the highest member is the Utica slate. 



The Potsdam is mostly an indurated sandstone or quart zite, 

 coarse and conglomeratic near the bottom. It lies along the entire 

 northern border but thins out nearly to disappearance to the south. 

 The Beekmantown, or Calciferous, formation following the Pots- 

 dam consists of calcareous sandstone and limestone and is found 

 on all sides except the western. In the Champlain region it attains 

 its extreme thickness. The Chazy limestone, which is next in order, 

 is confined to the Champlain valley. The Lowville, Black River 

 and Trenton formations are made up of gray and black limestones, 

 with shaly partings in the Trenton marking a transition into the 

 Utica^shale, the last of the series. They are mainly developed on 

 the south. 



