204 ROCHESTER ACADEMY OF SCIENCE. [June 22, 
gneissic rocks are dynamo-metamorphosed phases of eruptives and 
affords an additional argument In favor of this view. 
There is thus a reasonable probability that zircon, and to a less 
degree monazite, may prove to be guide minerals by which eruptives 
and their derivatives can be certainly identified, no matter what degree 
of alteration they may have suffered. This probability gives an 
additional interest to the study of the heavy residues of rocks which, it 
is hoped, will lead geologists to thoroughly test this hypothesis in other 
parts of the world. 
In conclusion, a word on the discrimination of monazite and 
xenotime may not be out of place, since the extremely minute grains 
in which they occur in the residues are often liable to be confounded 
with other minerals or with each other. Mr. Allen Dick, of London, 
has kindly examined some of my slides with the spectroscope and finds 
that the smallest grains can be readily distinguished by the absorption 
bands of didymium in monazite, and of erbium in xenotime. By 
bringing, by the use of condensers beneath the stage, the image of the 
sun, or of a small lamplight, zwztA#zn the grain and substituting an ordi- 
nary hand spectroscope for the eye-piece, the bands can be distinctly 
seen. 
APPENDIX. 
Since the above was written the receipt of a collection of American 
rocks from the National Museum of Washington has permitted a 
comparison of their heavy residues with those of the corresponding 
groups of Brazilian rocks. By chipping from the specimens, residues 
were obtained from 18 granites and g gneisses which, though too small 
for a complete study, were sufficient for a determination of the more 
abundant and characteristic accessories. 
Zircon was found in all, the rounded and sharp cut forms being 
about equally distributed in the granites and gneisses and frequently 
occurring together in the same specimen. In the gneiss of Ayer, Mass., 
and White Mountain Notch, N. H., the crystals are small and rare and 
in this respect are similar to those from the mica schist at the top of 
Mt. Washington, N. H., where their character is such as might 
be expected in a metamorphosed argillaceous sediment. They are, 
however, equally rare and small in some of the granites, particularly in 
that of Fairfield, S. C., and Somerville, Me., in the latter of which 
geniculated twins occur. The crystals are especially abundant and 
handsome in the granite of Otter Creek, Mt. Desert, Me., Hurricane 
Ils., Me., Ilchester, Md., and the gneiss of Endfield, N. H. and 
