9r»'> 
The America)! Geoloijist. October, is% 
this country, the origin seems to iiave been diH'erent from tliut 
of the Bedford gneiss. 
W. IT. Hohbs.* in his paper on the Schists of the Southern 
lierkshires, Mass., states that the large porphyritic feldspars 
are often filled with secondary granophyre and frequently 
show secondary enlargement. These porphyritic feldspars 
are more or less oval in shape and consist of an acid plagio- 
clase. The "augen" may show no marked evidence of strain, 
except polysynthetic twinning about cracks, and the grano- 
phyre structure seems to give evidence of being secondary and 
not therefore proving the igneous origin of the rock. Second- 
ary enlargement of the feldspars is very common and is re- 
garded as recrystallization of the detrital grains of the clastic 
rock, produced by static metamorphism (pressure, aided by 
heat and mineralizers ). 
The importance of the enlargement of mineral fragments in 
clastic rocks as a factor in their alteration by metamorphism 
has been emphasized by Irving and VanHise in their papers 
on the lake Superior region. 
C L. Whittle,! ^" '^^'^ work on the "Dynamic and Metamor- 
phic Phenomena in a Metamorphosed Conglomerate in the 
Green Mountains," states that the larger porphyritic crystals 
of albite {\ in. across and simple crystals or once twinned) in 
the chlorite schists at East Clarendon, Vt., were formed as 
secondary minerals by chemical solution and contain plenti- 
ful inclusions. A Assuring or faulting of the large albites 
was caused by a second period of mountain building forces. 
In N. H. Winehell's;!; paper, on the Origin of the Archjiean 
Greenstones, mention is made of the fact, reported b}'' G. H. 
Williams, that the feldspars have apparently withstood the 
pressure better than the quartz. The theory advanced by him 
is that the feldspars were produced bj^ the chemical action 
incident to the later dynamic forces and he calls attention to 
the fact that the feldspar is fresh in the most crushed portions 
of the rock. 
J. E. Woltf,J< in his paper on the Metamorphism of Clastic 
Feldspar in Conglomerate Schist, describes porphyritic albite 
*Bull. Geol. See. Amer., iv, 167, ~~~~~ 
tBull. Geol. See. Amer., iv, 118. 
:}:Geol. & Nat. Hist. Surv. Minn., Pt. II, 23il Ann. Kept. 
§Bull. Mus. Conip. Zool., xvi. No. 10. 
