212 B. K. Emerson— Northfieldite. 



Abt. XIX. — Northfieldite, Pegmatite, and Pegmatite Schist y 

 by B. K. Emerson. 



The band of granite gneiss, which runs up through Wilbra- 

 ham, Mass., and, interruped by the tonalite in Belchertown, con- 

 tinues north through Pelham to Northfield, is at its south end a 

 quite basic rock like the Monson band next east, but as it crosses 

 Pelham it is found by chemical and microscopic analyses to 

 become gradually a very acid one. The Monson rock in the 

 southern part contains 65 - 02 of silica, the Pelham gneiss in 

 Pelham 72'45 per cent of silica and in the northern area in 

 Erving 74*15 per cent. The Monson extends in a broad 

 many lobed mass across the state and has everywhere a wide 

 diorite border bed against the adjacent schists and very 

 generally an equally wide band occurs inside this border, 

 separating it from the normal granite, which is whiter and 

 often finer grained than the granite, often indeed developed 

 as an aplite. I have followed the diorite band 136 miles 

 around the Monson batholite in Massachusetts. The above 

 mentioned gneiss in Wilbraham to the west of the Monson 

 has the same diorite and aplite band but as it reappears in 

 Belchertown these bands become thinner and in Pelham the 

 diorite band thins out and disappears and the aplite is replaced 

 by a band of contact quartz rock which thickens and grows 

 coarser as it goes north and becomes in Crag Mountain over 

 300 feet thick. Its coarser varieties resemble a vein quartz, 

 or greisen, or pegmatite minus feldspar, its finer, a quartzite 

 like the Cambrian in the Berkshires. As a pegmatite or 

 aplite dike may pass into a quartz vein, the central mass of the 

 batholite seems here to pass on a large scale up into this quart- 

 zose border variant. 



"While the exterior resemblance is close, it is not wholly 

 satisfactory to apply the name greisen to this rock, since it 

 does not seem to be the result of later pneumatolitic changes 

 whereby the feldspar of the granite has been removed, but to 

 be rather an original ultra-acid contact differentiate and deposit 

 of the magma. Since it is a member of the unaltered plutonic 

 series and can not be called a vein quartz, a quartzite, or a 

 greisen, it is named here northfieldite, from the mass in North- 

 field forming Crag Mountain. 



The bedding of the Pelham gneiss forms a broad, flat arch, 

 horizontal over a wide central portion and with low dip to the 

 east and west along the east and west border, respectively. 

 The bedding of the northfieldite is conformable with that 

 of the granite-gneiss, and in many places the gneiss can 

 be seen to pass up into the northfieldite by easy transition. 



