GEOLOGY OF THE BROADALBIN QUADRANGLE 1 7 



This rock which, under the old classification, is a quartz-horn- 

 blende-syenite is, under the new classification, a hornblende-ada- 

 mellose. 



The high magnetite content of the measured sections calls for 

 more iron oxid than is shown in the analysis, but it must be re- 

 membered that the amount of magnetite varies notably even in very 

 short distances so that in the particular slides measured it runs 

 higehr than in the material analyzed. Also the titanium most 

 likely replaces iron oxid in the magnetite to form a titaniferous 

 magnetite thereby lessening the necessary amount of iron oxid. 



A comparison of this syenite with the granite porphyry and with 

 certain other Adirondack rocks is given in a table on page 21. 



GRANITE PORPHYRY 



The typical granite porphyry presents a striking contrast to the 

 typical syenite as regards texture, mineralogical composition, and 

 general appearance in the field. It is an unquestioned igneous rock 

 of gray to pinkish gray color, thoroughly gneissoid, homogeneous 

 in large masses, and intrusive into the Grenville. Three minerals 

 — feldspar, quartz, and biotite — are always prominent to the 

 naked eye. In the typical rock a finely developed porphyritic texture 

 never fails, the phenocrysts of feldspar, often an inch or more in 

 length, being more or less flattened out parallel to the foliation. 

 These phenocrysts are imbedded in a fine-grained mass of feldspar, 

 quartz and biotite. Often quartz also occurs in large crystals 

 (phenocrysts) which have been so thoroughly flattened out parallel 

 to the foliation as to present a decided leaf-gneiss effect. The 

 rock, in thin section, always shows evidence of severe crushing and 

 granulation. The granulation of the feldspar phenocrysts is often 

 visible to the naked eye. 



The features will perhaps be best brought out by a description 

 of the most representative rock from each of the areas shown on 

 the geologic map. What may be regarded as the most typical 

 granite porphyry of the quadrangle occurs in the area one and 

 one-half miles north of Northville and contains on the average 

 about: 20 per cent microperthite ; 20 per cent microcline; 12 per 

 cent orthoclase ; 10 per cent oligoclase to andesine ; 30 per cent 

 quartz ; 6 per cent biotite ; i per cent magnetite : and small amounts 



