GEOLOGY OF THE NORTHERN ADIRONDACK REGION 30o 



two led the writer, some years since, to propose the name " Danne- 

 inora formation," from Dannemora mountain in Clinton county 

 for these gneisses of the northern Adirondacks, the name to serve 

 unless equivalency with the Ottawa gneiss can be shown, in which 

 case that name should be adopted. Since, however, possible con- 

 fusion with a noted Scandinavian locality may result, the name 

 Saranac formation is suggested to replace it. The rocks are well 

 exposed along the river of that name, in Clinton county, and in 

 its near vicinity. 



Anorthosite. This was the first of the somewhat later, great 

 igneous intrusions which invaded the rocks already described 

 from below, breaking them up, pushing them aside or raising them 

 on its back, and inclosing great horses of them in many places. 

 The fact that it has not been so excessively metamorphosed as 

 the previous rocks is indicative that it is considerably younger 

 than they, as is the further fact that its character indicates that 

 it solidified at considerable depth, and that therefore the Gren- 

 ville sediments must have become buried under a considerable 

 thickness of later deposits, since worn away, before the intrusion 

 took place. 



The Adirondack anorthosite is found principally in one great 

 connected mass, seemingly one single intrusion, though this may 

 not be the case, occupying a great area in Essex and southern 

 Franklin counties, of rudely triangular shape with indented base 

 and blunted apex, the base at the north. The base is some 55 

 miles across, and the hight of the triangle some 40 miles, the 

 area of country involved being some 1200 square miles at a rude 

 approximation. Occasional small areas of other rocks, in part 

 Grenville or doubtful gneisses, in part later intrusives. are found 

 within it, but, mostly, it extends unbroken throughout. The in- 

 clusions, or horses, of various gneisses are most numerous near 

 the borders of the mass, and some great tongues of the outlying 

 rocks project into it. There are also a few outlying masses, 

 mostly of very small size, the Rand hill mass in Clinton county, 

 which has an area of some 4 square miles, being perhaps as large 

 as any. These may represent independent smaller intrusions, but 



