CHAPTER XI. 

 THE ERUPTIVE ROCKS. 



INTRODUCTION. 



The species of igneous rocks occun-ing- within the area of the crystal- 

 Hne schists are: 



1. Granite, in the strict sense, or biotite-muscovite-grauite, the most 

 widely distributed. 



2. Granitite, or Ijiotite-granite, generally })orphyritic. 



3. Pegmatite, or muscovite-granite. 



4. Albitic granite in secondary veins in the pegmatite, remarkable for 

 their content of rare elements. 



5. Aplite. 



6. Quartz-gabbro. 



7. Tonalite, or quartz-diorite, wholly or in part derived fi'om No. 7, and 

 with it forming the syenite of President Hitchcock. 



8. Diorite. 



9. Diabase. 



10. Cortlandite. 



Within a square twenty-five miles on a side, with Northampton at its 

 center and its eastern line along the foothills on tlie east side of the broad 

 Connecticut Valley, in Belchertown and Pelham, the coiuitry consists for 

 the most part of large areas of granitic rocks of the above types. Where 

 schists cross the region they are contorted and granite-impregnated, and rest 

 upon the granite in sepai'ate sheets, often of small size, or nan-ow bands, 

 and all, down to the smaller fragments, retain their dip and strike, even 

 when surrounded on all sides by the massive rock. 



A large portion of the area outlined above lies beneath tlie sands and 

 sandstones of the Connecticut Valley, and the line of Triassic erui)ti()ns 



