458 W. J. MILLER ADIRONDACK AXORTHOSITE 



some places, as in the Lake Placid quadrangle. For most part, however, 

 the gabbroid magma was too stiff to cross-cut, penetrate, break up, and 

 tilt masses of the Grenville strata in a manner at all comparable to the 

 later syenite-granite magma. 



Soon after the intrusion of the laccolith, femic minerals began to form 

 toward its outer portion and to precipitate, thus permitting the accumu- 

 lation of plagioclase in the upper levels of the magmatic chamber, but 

 below and within the chilled border phase. Though this idea of the set- 

 tling of femic minerals is much like that advocated by Bowen, my hy- 

 pothesis differs in two important respects : First, the anorthosite did not 

 form by settling of plagioclase crystals, and, second, there was no devel- 

 opment of syenite-granite residual magma over the anorthosite. In my 

 opinion, that portion of the magma from which the femic crystals settled 

 was at first very largel}^, or wholly, molten, and then very slowly crystal- 

 lized with continued precipitation of femic crystals until the magma be- 

 came too viscous to permit settling of crystals. In other words, the anor- 

 thosite practically as such was actually to a very considerable degree 

 molten, and it is not, as Bowen maintains, merely a mass of precipitated 

 plagioclase crystals never, to at least a considerable degree, molten as 

 such. It is not necessary to believe that there was any great amount of 

 settling of femic constituents unless we assume a very femic original 

 gabbroid magma, because fully 10 per cent of the femic minerals never 

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

 thosite. 



According to my view, the femic constituents did not settle througli 

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

 pothesis. Furthermore, only heavy femic minerals sank and not femic 

 minerals followed by plagioclase in a magma which must have become 

 increasingly viscous. These are, I believe, very important points. Re- 

 garding the settling of crystals in magmas in general, Iddings^^ says : 



"The difference in density between the ferromagnesian minerals and the 

 average magma is sufficient to cause them to fall through the action of gravity 

 if the liquid were very fluid. But such magmas are more or less viscous near 

 the point of saturation, the more siliceous ones exceedingly so. . . . The 

 absence of evidence of any appreciable amount of settling of crystals in most 

 solidified igneous magmas indicates that in these cases the viscosity of the 

 liquid was such that no considerable precipitation could take place or that 

 there were other hindrances." 



It is evident, then, that the magma in the Adirondack region, out of 

 which the femic constituents settled, must to the very last of the process 



«> J. p. Iddings : Igneous Rocks, vol. 1, 1907, pp. 270-271. 



