DIFFERENTIATION OF THE SYENITE-GRANITE MAGMA 247 



Again, in the same paper he proves that the normal syenite shows a 

 gradual transition into, a red granitic gneiss. He says : 



"Between this village (Natural Bridge) and Carthage, 10 miles west, the 

 country is occupied by a red (granitic) gneiss, rather fine grained, and not, 

 as a rule, strongly foliated. Starting from the normal gabbro (syenite) at 

 Natural Bridge and passing toward this red gneiss the natural expectation, 

 based on the appearance of the two rocks, would be to find the one cutting, or 

 superimposed on, the other ; but such is not the case. On the contrary, about 

 two miles west of the village the gabbro (syenite) undergoes a conspicuous 

 modification. It becomes gradually finer grained and gneissoid and changes 

 from gray to red. In other words, it passes gradually into the red gneiss, 

 which must therefore be regarded as a modified portion of the gabbro (sye- 

 nite)." » 



He says the transition zone is about half a mile wide. He then de- 

 scribes a similar transition, but where the red gneiss is much coarser 

 grained and contains considerable hornblende. 



These careful observations made upon large rock masses clearly estab- 

 lish the fact that here at least a whole series, ranging from gabbro 

 through syenite to red granite, has been produced by magmatic differ- 

 entiation. The writer believes this to be a capital point as applied to 

 the Adirondack region in general, for many more recent observations have 

 shown this sort of differentiation to be common. 



Observations of II. P. Cushing. — Gushing, in his description of the 

 Franklin County syenite, says : 



"All intermediate stages between granite and syenite may be found, so that 

 there is evidently a passage of one rock into the other. These granites are 

 either red or green in color, and while very quartzose and poor in dark sili- 

 cates these are the same as appear in the syenites, and the quartz has the 

 same spindle form." " 



In his description of the syenite of the Long Lake quadrangle, Cushing 

 writes : 



"The rock is exceedingly variable, much more so than is the anorthosite. 

 All the varieties grade into one another, so that any separation in mapping is 

 an arbitrary matter. . . . The variations of the rock are in two main direc- 

 tions. In the one case the dark-colored minerals increase in quantity at the 

 expense of the feldspar, garnet appears, and quartz diminishes and disappears. 

 The syenite passes into a monzonite and ultimately into a shonkinite. . . . 

 In the other direction the rock changes by increasing quartz. . . . The 

 feldspar also changes slightly and tends to becojne red instead of green, pro- 

 's ibid., pp. 282-283. 

 « H. p. Cushing : 18th Ann. Kept N. Y. State Geologist, 1898, p. 106. 



