120 how crops feed. 



The Stexttic Series 

 consisting chiefly of QiKvtz, Feldspar, and Amphibole. 



Syenite is granite, save that aipphibole takes the place 

 of mica. In appearance it is like gi-ainte; its color is usu- 

 ally dark giay. Syenite is a very tough and durable rock, 

 often most valuable for building purposes. The famous 

 Quincy granite of Massachusetts is a syenite. Syenitic 

 Gficiss and Ilor/tblende Schi^it correspond to common 

 Gneiss and Mica Schist, hornblende taking the place of 

 mica. 



The Volcanic Series 

 consisting of Feldspar, Amphibole or Pyroxene, and 

 Zeolites. 



Diorite is a compact, tough, and heavy rock, common- 

 ly greenish-black, brownish-black, or grayish-black in 

 color. It contains amphibole, but no pyroxene, and is 

 an ancient lava. 



Dolerite or Trap in tlie fine-grained vai-ieties is scarcely 

 to be distinguished from Diorite by the appearance, and is 

 well exhibited in the Palisades of the Hudson and the 

 East and West Rocks of New Haven. It contains pyrox- 

 ene in place of amphibole. 



Basalt is like dolerite, but contains grains of chrysolite. 

 The recent lavas of volcanic regions are commonly basaltic 

 in composition, though very light and porous in texture. 



Porpbyry, — Associated with basalt occur some feld- 

 spathic lavas, of which porphyry is common. It consists 

 of a compact base of feldspar, with disseminated crystals 

 of feldspar usually lighter in color than the mass of the 

 rock. 



Pumice is a vesicular rock, having nearly the composi- 

 tion of feldspar. 



TriE Magnesiax Series 

 consisting of Quartz, Feldsp>ar and Talc, or Chlorite. 



Talcose Granite differs from common granite in the 

 substitution of talc for mica. Is a fragile and more easily 



