The '■•Aiajen'" -Gneiss Area. — Lvquer and Hies. 253 
■) 
grains of variable size which are often twinned in two simple 
halves. They are clear and fresh and contain inclusions. 
The appearance of the clastic grains which have been en- 
larged is also described and the porphyritic development of 
the feldspar seems to be due to secondary enlargement or al- 
most complete recrystallization. 
Woltf* also mentions that the Stamford gneiss, whose ori- 
gin is still doubtful, contains large porphyritic twins of mi- 
crocline. It is evident that in this rock the development of 
the mica, quartz and feldspar, parallel to the planes of break- 
ing and sliding, has had a great deal to do with the parallel 
structure; hence it might have been an eruptive granite mod- 
ified by metamorphism, but the field relations point to a de- 
trital origin. An "augen"-gneiss, occurring in Hoosac moun- 
tain, and containing rounded feldspar crystals is also men- 
tioned by Wolff. In this instance the rocks have been crushed 
by great pressure, accompanied by shearing action, which 
has formed new feldspar, mica and quartz. 
As further instances of similar structure in rocks, but hav- 
ing an origin more like that of the Bedford ''augen"-gneiss, 
may be mentioned the occurrences described by Lehmann in 
his great work on "Die Entshehung der Altkrystallinischen 
Schiefer," in which there are given numerous examples of the 
production of "augen" by shearing forces, as illustrated in 
pi. X, fig. 5; XI, fig. 5, and XII, figs. I and 3, the latter espe- 
cially resembling the "auge'' figured from McDonald's quarry 
(see Plate IX,Fig. 5 of this work). Lehmann states (p.24:9) that 
the feldspars possess a special interest because of the excellent 
manner in which they show the result of pressure in bending, 
splitting, etc. He considers, however, that their surround- 
ings must have been very firm, as only under such conditions 
woultl the slightly pliable feldspars stand bending and stretch- 
ing, and he furthermore thinks that the cracks which they 
show, and which may or may not be filled with foreign mate- 
rial, preclude the possibility of the feldspar having been in a 
fused condition. 
(.'. ("allawayj- has described a mica-gneiss, from the Malvern 
hills, which has resulted from a granite by crushing. The 
*Mon. XXIII, U. S. Geol. Surv. "" 
tQuart. .Jour. Geol. Soc, xliu, .52."), 1887. 
