250 J.D. TRUEMAN 
glass. Derby" has noted this distinction and remarked that ‘‘a 
lack-luster aspect without evidence of alteration is the most certain 
sign of wear.’ This dull appearance of the grains is brought out 
fairly well in Fig. 7. 
While it is true that a fresh appearance, like good crystal form, 
may be preserved during sedimentation, the writer regards this 
feature as of considerable value as an additional proof of igneous or 
sedimentary origin. 
The significance of peculiar crystal forms.—The zircons in many 
rocks possess distinct individuality in their forms and the writer 
considers this peculiarity may be made use of in determining the 
origin of foliated rocks. It sometimes happens, for example, that 
near an area of schistose rocks there are other rocks of less altered 
character which, according to field relations, might well represent 
part of the original rock, now largely schistose. It is the writer’s 
opinion that zircons in the fresh and altered rocks might be so 
similar in character as to be fairly conclusive evidence that the two 
rocks were originally of the same character. 
The variation in form depends largely upon the relative develop- 
ment of the different possible faces. Sometimes the zircons are 
needle-like in character with the prismatic faces prominent, and 
at other times the crystals may be short and terminated by one or 
more sets of pyramids. The basal pinacoid face seems to be only 
rarely developed? but is present in the majority of grains in the 
section made from the Butte granite and illustrated in Fig. 8. 
This use of peculiar crystal forms has been tested by the writer 
successfully in several cases where the fresh and unaltered repre- 
sentatives of a rock were obtainable. 
CONCLUSIONS REGARDING THE USES OF ZIRCON AS A CRITERION 
1. The presence of abundant, minute grains of zircon in a meta- 
morphosed rock strongly indicates that the original rock was either 
igneous or an arenaceous sediment. A recomposed igneous rock 
could in no way, of course, be distinguished from an igneous one 
by means of this mineral. The possibility of introduction from 
1 Proc. Rochester Acad. Sci., I (1891), 202. 
2 J. D. Dana, Descriptive Mineralogy (19090), 483. 
