GEOLOGY OF THE SCHROON LAKE QUADRANGLE 37 



country rock (Grenville) and, at first, when it was in its most 

 highly fluid state, attacked the country rock with sufficient force 

 to engulf portions of it, send some dikes into it, and even inti- 

 mately penetrate it. Such phenomena have been observed in the 

 Lake Placid and the Schroon Lake quadrangles. For most part, 

 however, the gabbroid magma was too stiff to cross-cut, penetrate, 

 break up and tilt masses of the Grenville in a manner at all com- 

 parable to the later syenite-granite magma. 



Soon after the intrusion of the laccolith, many of the femic 

 minerals which began to form in the magma just below and within 

 the chilled border phase began to precipitate, thus permitting the 

 accumulation of plagioclase in the upper levels of the magmatic 

 chamber. Though this idea of settling of femic minerals is much 

 like that advocated by Bowen, the writer's hypothesis differs in two 

 important respects. First, the anorthosite did not form by settling 

 of plagioclase crystals, and, .second, there was no development of 

 syenite-granite residual magma over the anorthosite. It is not 

 necessary to believe that there was any great amount of settling of 

 femic constituents unless we assume a very femic original gab- 

 broid magma, because femic minerals up to fully 10 per cent never 

 precipitated at all, these being now present in the typical Marcy 

 anorthosite. Further, the femic constituents did not settle through 

 anything like such a thick mass of magma as required by Bowen's 

 hypothesis, and only heavy femic minerals sank, and not femic 

 minerals followed by plagioclase in a magma which must have 

 become increasingly viscous. 



It is evident, then, that the gabbroid magma in the Adirondack 

 region must, to the very last of the process, have contained at least 

 a very considerable percentage of liquid of sufficient fluidity to 

 have allowed the crystals to sink through, and this residual magma 

 is represented by the present anorthosite, which was once to a 

 very considerable degree molten as such. Could the magma have 

 possessed sufficient fluidity long enough to have permitted repeated 

 wholesale sinking of crystals through a very great thickness of 

 magma as required by Bowen's hypothesis? 



Origin of variations in the anorthosite. As already shown, the 

 anorthosite -contains many zones or belts and irregular-shaped por- 

 tions, some distinctly more gabbroid, and others distinctly more 

 highly feldspathic than the average Marcy anorthosite. Many of 

 these show relatively wide gradation zones into the typical rock, 



