246 W. J. MILLER MAGMATIC DIFFERENTIATION AND ASSIMILATION 



GRANITIC SYENITE 



By increase in quartz content to from 20 to 25 per cent the normal 

 syenite passes into what may be called granitic syenite. Microcline often 

 occurs in considerable quantity. Biotite usually occurs in amounts up to 

 8 or 10 per cent, and sometimes it appears to be secondarily derived from 

 hornblende. Pyroxene has not been noted in this rock. The color varies 

 from gray or greenish gray to pink or almost red, this red color appar- 

 ently being due to hematite specks or stains. In all other respects the 

 granitic syenite is much like the normal syenite, 



GRANITE AND GRANITE PORPBYRY 



The granite and granite porph3ay represent the most acidic phases of 

 the great syenite-granite body, and these show perfect gradations through 

 granitic syenite to normal syenite. Arbitrarily the rocks are called gran- 

 ite when the quartz content exceeds 25 per cent. 



The colors range through greenish gray, light gray, pink to almost red, 

 with pink or red granites most common. Compared with the normal sye- 

 nite, these granites have almost invariably smaller amounts (usually only 

 5 to 10 per cent) of dark-colored minerals ; frequently considerable 

 amounts of microcline, in addition to microperthite, orthoclase, and 

 plagioclase; much more quartz; generally less hornblende or none; no 

 pyroxene, and almost constant presence of biotite (or muscovite) up to 

 15 or 16 per cent. In all other respects the granite is like the normal 

 syenite or granitic syenite, while the granite porphyry differs from the 

 granite only in its very distinct porphyritic texture. 



The so-called '^Laurentian granite" of the Thousand Islands region is 

 in no essential particular different from the granite here described. 



Differentiation of the Syenite-Granite Magma 

 actual examples 



Observations of C. H. Smyth, Jr. — In an important paper dealing with 

 the rocks of the western Adirondack region, Smyth says, concerning the 

 syenite (called gabbro^ in this paper) : 



"Different specimens of the rock require, from a strictly petrographic point 

 of view, different names, ranging from gabbro and anorthosite to augite-sye- 

 nite. . . . They are different portions of a single intrusive mass and owe 

 their differences to magmatie variations." * 



3 In later papers by Smyth the rock is called syenite instead of gabbro, and It Is now 

 known to be identical with the great bodies of syenite so common in the Adirondack 

 region. 



* C. H. Smyth, Jr. : Bull. Geol. Soc. Am., vol. 6, 1895, pp. 27i and 274, 



