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PRIMARY ROCKS. 



CHAPTER XX. 

 GEOLOGY OF THE UNITED STATES. * 



PRIMARY ROCKS OF THE UNITED STATES. 



Primitive Rocks —Prof. Hitchcock's Arrangement.— Stratified 

 and Unstratified. — Distribution of Primary Rocks, — In New- 

 England. — In the Middle States. — Syenitic Porphyry.— Pri- 

 mary Stratified Rocks. — Gneiss. — Mica Slate. — Talcose 

 Slate. — Granular Limestone. — Minerals in Granitoid Rocks. 



In treating of the geology of the United States, 

 we shall follow the same arrangement which we 

 have adopted in the forepart of our treatise, and 

 describe the different formations in the following 

 order. 1. Primary Rocks; 2. Transition Rocks; 3. 

 Secondary ; 4. Tertiary; 5. Basaltic and Volcanic; 

 6. Alluvial and Diluvial, Some geologists begin at 

 the surface and go down ; we prefer to commence 

 at the bottom, with what have usually been called 

 the primary rocks, and work our way upward. 



Professor Hitchcock* has arranged the prim- 

 itive rocks of Massachusetts chiefly under three 

 groups, as follows : the first group embraces gran- 

 ite, syenite^ greenstone^ and porphyry : the second^ 

 gneiss^ hornblende slate^ and quartz rock: the thirds 

 mica slate, talcose slate, chlorite slate, and hornblende 

 slate. A fourth group embraces limestone, scapoliie 

 rock, steatite, and serpentine, which may be called 

 miscellaneous rocks. The student of geology will 

 find it advantageous to furnish himself with speci- 

 mens of all these rocks ; but, when he goes abroad 

 to observe, he will find that granite, gneiss, and mica 



* Report on the Geology of Massachusetts. 



