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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 third, 
mica slate, talcose slate, chlorite slate, and hornblende 
slate. A fourth group embraces limestone, scapoliie 
rock, steatite, and serpentine, which may he 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 
10 observe, he will find that granite, gneiss, and mic& 
* Report on the Geology of Massachusetts. 
