TRANSITION ROCKS 451 



to Bowen, formed between the syenite and anorthosite should be poorer 

 in femic minerals than the anorthosite. JSTow, in my experience, jnst the 

 reverse is true. The average syenite is notably, and the average transi- 

 tion rock (Keene gneiss) is perceptibly, richer in femic minerals than 

 the anorthosite. This is a fact overlooked by Bowen. Why were more 

 femic minerals left in the overlying syenite and transition rock magmas 

 than in the underlying anorthosite, according to his view, when the ten- 

 dency was for such crystals to go to the bottom? This serious difficulty 

 is entirely obviated if we consider the anorthosite to be a separate instru- 

 sive distinctly older than the syenite-granite series. 



SIGNIFICANCE OF THE THICKNESS OF THE KEENE GNEISS 



The thickness of the Keene gneiss is very significant. It almost always 

 occurs as narrow belts or zones between syenite or granite and anortho- 

 site. A very fine example is the narrow belt forming a transition between 

 syenite and typical Marcy anorthosite across the Sentinel Eange of the 

 Lake Placid quadrangle. If we consider its thickness to be approximately 

 represented by the width of the outcrop, it would be but little more than 

 a thousand feet; and this is my view. But, according to Bowen's hy- 

 pothesis, this Keene gneiss must extend as a layer southward under the 

 syenite, aiid hence its thickness would be much less ; in fact, scarcely more 

 than a few hundred feet. Now, within this short distance the passage 

 from typical quartz syenite into typical Marcy anorthosite takes place. 

 The Marcy anorthosite is certainly at least some thousands of feet thick, 

 several thousand feet in thickness being exposed in single mountains, and 

 so with the syenite. I can not conceive of the development of two rock 

 masses, syenite above and anorthosite below, on such a grand scale by the 

 sinking of plagioclase crystals in a slowly cooling magma under deep- 

 seated conditions, as required by Bowen's hypothesis, without the forma- 

 tion of a notably thick transition rock, fairly comparable in bulk to the 

 overlying syenite and the underlying anorthosite. But this is far from 

 the case as demonstrated by the field relations. As matter of fact, as 

 already pointed out, such a transition rock never formed at all in some 

 places. 



General Absence of Grenville and Syenite-granite from the 

 Anorthosite Area 



It is a striking fact that both Grenville and syenite-granite are almost, 

 if not entirely, absent from a large part of the area of anorthosite. 



