r46 NEW YORK STATE MUSEUM 



is well exposed near the road from Paul Smiths to Brandon, in a 

 field just north of the road on Hall's farm, about a mile west of 

 Keese's Mills. 



The other rock (936) is augite syenite of the same type as that 

 just described (935), though here its field relations to the anortho- 

 site are not exhibited. It has a somewhat larger content of dark 

 silicates, and quartz is practically absent, but the feldspars are 

 of the same peculiar intergrowth type. It is unquestionably the 

 same rock and in all probability intrusive into the anorthosite, 

 as that is. 



South of these exposures, along the river at Tromblee's and 

 between^ are ridges with many exposures, all seen being of 

 anorthosite gabbro. 



Summary of section. Though in this section lack of exposures 

 prevents ascertaining the possible existence of gradations be- 

 tween the anorthosite and syenite, the cutting of the anorthosite 

 by syenite of precisely the same type as that found in the previ- 

 ous sections is well shown; and, while in those only occasional 

 small dikes have been forthcoming, here the rock is found invad- 

 ing the anorthosite in considerable force. The evidence of this 

 section and the preceding as well is demonstrative that there is 

 in the region a considerable body of augite syenite that grades 

 into the anorthosite, but that also was, as a whole, slightly later 

 in cooling, so that it has invaded the main body of the anortho- 

 site in the form of dikes. The presence of these dikes would 

 seem fatal to the theory that the intermediate rocks were pro- 

 duced through the absorption, by the anorthosite magma, of a 

 certain amount of an earlier syenite, since it is difficult to under- 

 stand how the border rock thus produced could remain molten 

 longer than the parent mass. The phenomenon would be ex- 

 plicable however on the assumption that the syenite was the 

 younger, and that the intermediate rock was produced by the 

 melting away and absorption of a certain amount of the anortho- 

 site. Whether the intermediate rock has been formed in this 

 fashion or by direct differentiation is uncertain, but in either 

 case the later date of the syenite is a necessary conclusion. 



