CONTACT EFFECTS OF THE EEUPTIVE ROCKS. 349 



pegmatite dikes. The circuiustauces uuder which a crystal of spodumeue 

 a yard long aud a foot thick could form in a great granite dike and then 

 be replaced by albitic granite containing zircon, garnet, and beryl are diffi- 

 cult to imagine. It is, perhaps, possible to suppose that tlie latter minerals 

 were included in the original crystal, and then the change l)y the action of 

 heated alkaline solutions as made out by the authors cited above (p. 344ff) 

 seems satisfactory for the explanation of the main change into albitic granite. 

 It seems to me that the succession made out above — (1) tonalite and 

 granitite, (2) pegmatite, (3) albitic granite — was essentiall}- a series of 

 eruptions in which mineralizers took a gradually inci'easing part, and that 

 aqueous agency proper began with the formation of cymatolite and the 

 other remarkable pseudomorphs and the quartz veins. 



COlSTTACT EFFECTS OF THE ERUPTI\t: ROCKS. 



These rocks penetrate highly crystalline schists and gneisses, and in 

 general the contact effects are not marked. On the west side of the valley 

 the complex spangled structure of the Conway schists disappears, the trans- 

 verse biotite and garnets are wanting, and the rock is coarser-grained and 

 feldspathic. On the east side it becomes a coarse fibrolite-gneiss. In the 

 amphibolites and argillites the changes are more interesting. The broad 

 band of chiastolite-schists derived from the Leydeu argillite is described in 

 connection with the description of these rocks. The others are discussed 

 also in connection with the less altered rocks with which they are asso- 

 ciated and from which they have been derived, as the purpose of this study 

 has been to determine the sequence aud proper association of the crystalline 

 schists. 



Around the l^order of the Belchertown tonalite, and to a less extent 

 around the Hatfield area, are dark-green, friable, granular pyroxene rocks, 

 which represent, apparently, an eifect of contact metamorphism. (See p. 

 243.) Near the western border of the former mass, back of the house of 

 T. S. Haskel, the rock appears near a great dike of pegmatite. 



Also across the river, near the north, end of the western exposure of the 

 tonalite, occurs a biotite- and pyroxene-bearing rock, greenish-black and 

 somewhat above medium grain. Abundant large scales of biotite give it 

 a shining appearance, and the green granular pp-oxene is often visible, 

 and the microscope shows the finest regular cross-sections, with well- 



