GEOLOGY OF OGDENSBURG 15 



foliated mica schists there are all gradations into a solid, massive 

 black rock, fairly coarse grained, and with a poor foliation, which 

 is a rather typical amphibolite. 



The amphibolites found in the Adirondack Precambrian look 

 much alike, but have originated in at least three quite different 

 ways and from rocks which were originally unlike. In part they 

 represent metamorphosed calcareous shales, which occur in beds 

 within the Grenville series and are an integral part of that series. 

 They are also produced from somewhat impure limestones as a 

 result of the contact action of granitic intrusion upon these lime- 

 stones. Such amphibolites are found only at such contacts, and 

 should show a gradation back into limestone as we recede from the 

 contact. In still other parts the amphibolites are produced from 

 original basic igneous rocks, gabbros, due to recrystallization under 

 the conditions of heat and pressure which have so profoundly 

 changed all these old rocks from their original condition. 



There are probably amphibolites of all three origins within the 

 Ogdensburg quadrangle, but much uncertainty prevails in regard 

 to most of the occurrences. The belt of schists at the eastern 

 margin of the sheet, between the granite and the Potsdam, consists 

 of a jumble of thin beds of many sorts of rock, much folded and 

 contorted and following one another in rapid succession. Thin 

 limestones and quartzites are interbedded with impure limestones, 

 garnet schists, amphibolites and other schists in such wise that 

 they could be separately mapped only on a very large scale map 

 with the aid of multitudinous exposures. The amphibolites here, 

 however, seem certainly to be metamorphosed shales. 



Midway of the southern margin of the Ogdensburg sheet is a 

 broad belt of Grenville amphibolite much involved with porphyritic 

 granite. The granite occurs in long, narrow tongues in the amphi- 

 bolite, but these hold many inclusions of amphibolite. The amphi- 

 bolites are cut by a multitude of narrow dikes from the granite, 

 and in places are much granulated and soaked by the granite, with 

 the production of mixed rocks. The granite also carries much black 

 mica, and is thoroughly foliated, so that, in the places where the 

 porphyritic crystals of feldspar fail, it looks not unlike some phases 

 of the amphibolite. The main granite tongues can be mapped, and 

 the chief amphibolite masses are readily differentiated from them, 

 but there remains much mixed material, here mapped with the am- 

 phibolites, which forms a very puzzling combination. The amphi- 

 bolite itself also varies greatly. The central part of the mass, that 



