F. Ward — Lighthouse Granite near New Haven, Conn. 



39 



The Pegmatite. — The minerals of the pegmatite are, as in 

 the granite, chiefly quartz and feldspar, with a little mica and 

 some magnetite. The mica is nearly all biotite ; in some cases 

 it is segregated somewhat along the margins of the dike type. 

 There seems to have been no unusual pneumatolytic action, at 

 least there are no minerals present which are uncommon in 

 granite — not even tourmaline. At one place a little ilmenite 

 was found with the magnetite. 



This non-mineralized character of the pegmatite is a striking 

 fact. It is most easily explained by simply saying that this 

 particular magma did not possess any gases containing boron, 

 fluorine, etc., and so such minerals as tourmaline, topaz, beryl, 



etc., could not form. But it is also possible that there might 

 have been certain zones throughout the magmatic mass which 

 did not carry pneumatolytic minerals and other zones that did. 

 Such zonal arrangement or distribution might arise from the 

 fact that under conditions of high temperature and pressure, 

 such as would exist in the parent magma, these rarer gases 

 (perhaps in some other active physico-chemical form) would 

 act rather as solvents, with the result that no deposition or 

 formation of any minerals containing them could take place. 

 But such gases, laden with other elements, on passing up through 

 pegmatitic channels, would eventually reach regions where the 

 temperatures and pressures were much lower; here they could 

 no longer act as solvents, but, by combining with various 

 elements and by interacting with other materials present, 



