PEGMATITE, SILEXITE, AND APLITE OF NEW YORK 29 



except magnetite, are either almost wholly absent, or they make 

 up less than 5 per cent of the rock. Thin sections of the typical 

 granites contain microperthite, 45 to 70 per cent; quartz, 25 to 

 35 per cent; oHgoclase, 5 to 25 per cent; usually microcline, 5 

 to 15 per cent; magnetite, ^ to 2 per cent; and usually less than 

 2 per cent of one or more of the following: titanite, hornblende or 

 pyroxene, biotite, zircon, apatite, and hematite (stains). Locally, 

 probably owing to incorporation of material from older basic 

 rocks, dark minerals, including hornblende, monoclinic pyroxene, 

 and biotite, are fairly conspicuous. A magmatic flow-structure 

 foHation is generally more or less well developed in the granite, 

 but some portions of the rock which are almost free from dark 

 minerals exhibit practically no foliation to the naked eye. 



The writer proposes to call the rock just described the "Lyon 

 Mountain granite" because of the excellent exposures in and 

 near the village of Lyon Mountain. This rock is also extensively 

 developed on Johnson Mountain, Duncan Mountain, and in the 

 valley of the Saranac River. The Lyon Mountain granite is 

 perhaps the most conspicuous member of Cushing's "Saranac 

 formation."^ This Lyon Mountain granite contains a truly won- 

 derful display of pegmatite and quartz (silexite) in the form of 

 dikes, segregation masses, and inclusions. In many places the 

 granite is very highly pegmatized. All the numerous magnetic 

 iron-ore deposits, including those of the Lyon Mountain mines, 

 occur in this granite. The structure and origin of these deposits 

 are discussed by the writer in another paper. ^ 



There is considerable field evidence to show that the Lyon 

 Mountain granite grades into and is only a facies of a coarse- 

 grained rock which the writer proposes to name the " Hawkey e 

 granite" because of its excellent outcrops just east of Hawkeye 

 post-office. In thin section the rock from the type locality shows 

 microcHne, microperthite, and quartz in about equal amounts; 

 8 to 10 per cent of oHgoclase to albite; and small quantities of 

 biotite, apatite, zircon, magnetite, chlorite, and secondary calcite. 



" H. P. Gushing, N.Y. State Mus., Bull, pj (1905), pp. 299-303. 

 'W. J. Miller, N.Y. State Mus. Bull., on the geology of the Lyon Mountain 

 quadrangle soon to appear. 



