250 J.D. TROEMAWN 
The capacity of zircon to resist alteration.—To one who cannot 
‘see any virtue in the use of zircon as a criterion, if this mineral 
can be formed in a rock secondarily, the evidence given in the last 
section must certainly be disappointing. If the reader, however, 
is willing to accept a compromise and appreciate the value of a 
mineral as a criterion which is markedly characteristic of certain 
rock types, widely distributed, and which remains practically 
unaltered in a rock long after the commoner constituents have 
Fic. 6.—Rib Hill quartzite. Crossed nicols. 20 
entirely recrystallized perhaps there is no reason why he should 
not regard the method as one of distinct promise. 
To illustrate: Take the case of the Wausau quartzite which 
contained the zircons with sécondary enlargement. The quartz 
has here undergone such extreme recrystallization that ordinary 
evidence of clastic texture has been entirely destroyed. The rock, 
indeed, is almost as vitreous as ordinary vein quartz. A micro- 
photograph of a section of this rock is shown in Fig. 6. Fig. 7 
represents zircons taken from the same specimen as the slide 
illustrated in Fig. 6 and shows their round and well-worn appear- 
ance, a feature which, as will be seen later, is more or less character- 
