348 GEOLOGY OF OLD HAMPSHIRE COUNTY, MASS. 



AGE OF THE GRAKITES. 



The tonalite cuts the Leyden argillite, one of our newest rocks, in 

 Hatfield. A porphyritic and a common granite dike cut the same argillite 

 on the north line of the State, and 3 miles south quartz-muscovite veins in 

 the Bernardston Devonian limestone seem to me to be the outermost fila- 

 ments of the same granitic intrusion. The pegmatite cuts the tonalite. On 

 the other hand, the way in which the dikes are intruded tjetweeu the layers 

 of the vertical schists would indicate that this intrusion occurred after their 

 upfolding, while the way in which the tourmalines are broken in pieces and 

 the great spodumene crystals are bent and many times fractured and 

 faulted woidd indicate that some part of the folding has been done since 

 they reached their present position. We may, then, consider the tonalite 

 to be the oldest, the pegmatite and cortlandite the newest, and the whole 

 series as of late Devonian or Carboniferous age. The porphyritic granitite 

 seems to be older and to have been influenced more by the upfolding of the 

 region, and the Hardwick granite is still older, as it is cut by the porphy- 

 ritic granite of Coys Hill. 



R:6sUM]6 as to THE GEKETIC RELATIONS OF THE GRANITES. 



The two great masses of tonalite were the cores of two batholites, which 

 came up at points on the two faults which border the great Connecticut 

 River depression. 



From the northeast and northwest corners of the Belchertown mass 

 extend the eastern Connecticut and the Swift River fault lines, along which 

 narrow dikes and patches of the tonalite occur far to the north. 



Unlike the above, the Hatfield tonalite is bordered outwardly in its 

 western half by biotite-granite (granitite) and biotite-muscovite-granite, 

 and then both are much cut up by later pegmatite dikes in several genera- 

 tions, which extend out into the suiTounding country in a broad aureole, 

 within and bevond which the schist is greatly impregnated with quartz and 

 considerably more metamorphosed than outside their influence. 



Toward the periphery the granite dikes carry rare minerals in great 

 abundance and beauty, and these show two modes of occurrence. Beryl 

 and large manganese-ganiets occur irregularly in the muscovite-granite 

 dikes of very coarse grain. Albite, tourmaline, and the minerals of the 

 rare earths occur in secondary dikes of most puzzling character in the main 



