180 



STRUCTURAL GEOLOGY OF NORTH AMERICA 





Fig. 11.25. Evolution of the syenite-granite stock of Ascutney Mountain, Vt. Reproduced from 

 Chapman and Chapman, 1940. 



jection. Notable of these are the Kinsman quartz monzonite and the 

 Bethlehem gneiss. 



According to Billings (1945): 



The Mt. Clough pluton, composed of Bethlehem gneiss, is undoubtedly the 

 longest intrusion in New Hampshire. The main body extends southward for 90 

 miles from the northern part of the Franconia quadrangle to the south end of 

 the Lovewell Mountain quadrangle, which is beyond the limits of Fig. 11.20. 

 The width ranges from half a mile to 7 miles. In the Moosilauke quadrangle the 

 contacts are essentially vertical and the pluton is a vertical sheet. Further south, 

 however, the contacts dip to the east and along the eastern border of the Mas- 

 coma quadrangle and the western border of the Cardigan quadrangle, the upper 

 and lower contacts dip 30 degrees east. Here the pluton is a huge sheet inclined 

 to the east [Fig. 11.26]. 



A series of plutons composed of Kinsman quartz monzonite lie east of the 

 Mt. Clough pluton. The most northerly of these, which may be called the Kins- 

 man pluton ... is a gigantic lens, essentially vertical in the surrounding schists. 



In the western part of New Hampshire, some ten miles east of the Connecti- 

 cut River, the crest of a major anticline is occupied by a series of "domes." In 

 their essential features these domes, nine of which have been mapped, are re- 

 markably similar. A central oval-shaped core of plutonic rocks, ranging in com- 



position from granodiorite through quartz monzonite to granite, has a foliation 

 that dips outward. The plutonic rocks, overlain by Ordovician (?), Silurian, 

 and Devonian strata, include the Ordovician (?) rocks in many localities and 

 the Silurian rocks in at least one locality. The upper contact of the plutonic 

 rocks is at essentially the same stratigraphic horizon in all the domes, approxi- 

 mately 500 feet below the top of the Ammonoosuc volcanics, but ranges from 

 the top to an horizon 1,000 feet below the top. The overlying formations like- 

 wise participate in the domical structure. 



Originally considered to be laccoliths or "bottomless" plugs that had bowed 

 up their roof, it is possible that they all belong to a single great concordant 

 sheet, originally horizontal, that has been buckled up during orogeny. 



Tectonic History 



Ordovician Sedimentation. The oldest rocks known so far in the eu- 

 geosyncline of New Hampshire are Middle Ordovician limestone, cal- 

 careous shale, and shale, 7000 to 8000 feet thick. Over these accumulated 

 the Upper Ordovician Ammonoosuc volcanics, about 4000 feet thick, and 

 over the volcanics another 500 to 2000 feet of shale. 



Taconic Orogeny. Near the close of Ordovician time the previously 

 deposited sediments and volcanics were mildly folded and eroded. The 

 disturbance here probably marks the subdued effects of die Taconic 

 orogeny of the Hudson-Champlain region farther west. 



Silurian and Lower Devonian Sedimentation. In a Middle Silurian 

 sea that moved in from the southwest, conglomerates and sands of the 

 Clough formation and the dolomitic sandstones and shales of the Fitch 

 formation, not over 800 feet thick, were deposited. Late Silurian history 

 is obscure, but during early Devonian time about 10,000 feet of sand- 

 stone, shale, and volcanic materials accumulated. See upper left section 

 of Fig. 11.27. 



Acadian Orogeny. During Mid- or Late Devonian, die strata were 

 caught in a major orogeny. Even before the deformation, or at least in 

 its early stages, successive injections of the Oliverian magma series 

 formed a great sheet in the Ammonoosuc volcanics, later to be domed 

 in several places along the western margin of New Hampshire. The 

 Ordovician, Silurian, and Devonian strata were thrown into a series of 

 anticlinoria and synclinoria whose axes trend north and northeast, and 

 countless minor folds were impressed upon the larger. Also the Northey 



