108 HENRY S. WASHINGTON 



composition of the dike as a whole is remarkably similar to that 

 of the granite of the region (analysis IV) , the only noteworthy 

 difference being in the soda. 



Quartz-syenite -porphyry . — Dikes of this are met with in con- 

 siderable numbers in the granite areas, especially that of Cape 

 Ann, where they have been mapped by Shaler. 1 The best speci- 

 mens in my possession come from the dikes numbered by Shaler 

 52, 53, and 70 on Eastern Point, and 245, a short distance north 

 of Squam Light, the last being the freshest. Megascopically 

 they show phenocrysts of alkali-feldspar, hornblende and 

 biotite, and fewer of quartz ; scattered through a fine-grained 

 groundmass composed of feldspar, some quartz and specks of 

 ferromagnesian minerals. The specimen from near Squam 

 Light is a rather dark ash-gray, while the others are brownish- 

 gray. 



Under the microscope the large feldspars are seen to be 

 microperthitic and are all cloudy and somewhat decomposed. 

 The large hornblendes are ragged in outline and usually slightly 

 altered, brown limonitic flakes being seen at their edges. They 

 are of a peculiar, rather vivid grass-green, resembling that of actin- 

 olite, and are quite pleochroic. A few ragged plates of greenish- 

 brown biotite are seen as phenocrysts. The groundmass is 

 granitic and holocrystalline, the quartzes clear and generally 

 interstitial between the feldspars. These are almost entirely of 

 orthoclase or soda-orthoclase, not usually microperthitic, with a 

 few doubtful oligoclases. They show a marked tendency to 

 automorphic development. Irregular shreds of hornblende and 

 biotite, similar to those forming the phenocrysts, are quite 

 common, the former being the more abundant. A few small 

 grains of magnetite and titanite are seen here and there and 

 minute needles of apatite are fairly common. 



For analysis the freshest specimen from Dike 245, north of 

 Squam Light, was chosen, the result being given below, with 

 that of the Wolf Hill nordmarkite for comparison. 



'Shaler, Ninth Ann. Rep. U. S. Geol. Surv. Plate LXXVII. 



