SOME UNUSUAL ROCKS FROM MAINE' 



EDSON S. BASTIN 



I. PROWERSOSE FROM KNOX COUNTY 



This unusual rock was collected by the writer in the summer of 

 1905 near Burkettville post-office in Knox County, Maine. It 

 outcrops over- a nearly- circular area about 3I to 4 miles across, 

 which lies mainly in the town of Appleton, but extends a short dis- 

 tance west into the town of Washington, and north into the town of 

 Liberty (Waldo County). It intrudes pehte schists and gneisses, 

 and is itself cut by numerous dikes of pink aplitic granite. Bowlders 

 of this rock had previously been observed 20 to 25 miles to the south- 

 east, near the villages of Sprucehead and St. George. 



In the field the rock was called a syenite-porphyry. The com- 

 monest phase is massive and shows numerous purplish-gray pheno- 

 crysts of feldspar in a rather fine, dark-green, even-grained matrix 

 made up almost entirely of biotite and- green hornblende (Fig. i). 

 The feldspar phenocrysts vary in length from £to i| inches, i inch 

 being about the normal length. A series of; measurements on 

 three hand-specimens showed that they constituted about 42 per 

 cent, of the rock. Many are twinned according to the Carlsbad 

 law, and some show zonal structure very perfectly brought out 

 through concentric alternation of yellowish-gray with purplish- 

 gray layers. 



Microscopic study shows most of the feldspar to be a perthitic 

 intergrowth or a somewhat irregular interpenetration of orthoclase 

 or microcHne, and albite. Microcline and orthoclase may or may 

 not be present in the same crystal. In general, the potassic feld- 

 spar dominates greatly over the sodic, so that megascopically no 

 albite twinning is visible in any of the phenocrysts. Some of the 

 smaller phenocrysts are pure microcline, and a very few are pure 

 orthoclase. Albite, not intergrown with the potash feldspars, 



^ Published with the permission of the Director of the U. S. Geological Survey. 



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