202 ROCHESTER ACADEMY OF SCIENCE, [June 22, 
altogether, though the observations on this point are too scanty for a 
definite conclusion. 
The successful application of this method of study of the heavy 
residues to rocks altered by decomposition, naturally suggests the 
hypothesis that it may also be of value in the investigation of those 
that have suffered alteration by metamorphism. Some of the crystal- 
line schists are now generally admitted to be derived by dynamo- 
metamorphism from eruptives ; others are as generally conceded to be 
metamorphosed sedimentaries, while the genetic relations of the 
greater part are still in dispute. Those of the first group should afford 
residues containing the characteristic accessories of the original 
eruptive type, either unchanged or in a secondary form, with, perhaps, 
others that have been produced in the process of metamorphism. 
Those of the second group, on the contrary, should only afford such 
accessories as may have existed as transported fragments in the original 
sediments, or as are susceptible of being produced during the met- 
amorphic process ; the former can generally be recognized by the 
evidence of wear that they present,* while the latter are well known 
through the studies on contact metamorphism, 
The minerals developed in undoubted metamorphosed sedimen- 
taries are the same that occur in the eruptives, and although the two 
groups present differences in the aspect, association and _ relative 
abundance in the constituents, no certain rule has yet been laid down 
by which they can be distinguished. Certain silicates, like staurolite, 
andalusite, etc., are generally regarded as more characteristic of 
metamorphosed sedimentaries than of eruptives, but as they have been 
occasionally reported from the latter, and as there is no apparent reason 
why they should not so occur they cannot be taken as guide 
minerals. If any such guide minerals are to be found, they must 
be looked for among those containing the rare chemical elements. 
The almost universal distribution in eruptives of zircon, with its 
frequent associates, monazite and xenotime, sbow that the rare elements 
zirconium and the various members of the cerium and yttrium groups 
occur, the first almost universally, the others frequently in eruptive 
magmas. The usual combination is that of the minerals above 
mentioned, which only exceptionally (so far as my experience goes 
only in the comparatively rare potash granites) suffer alteration in the 
decomposition of the parent rock. Owing to their generally minute 
size and high degree of hardness they also escape any considerable 
*Generally, but not universally, since very hard minerals like zircon, are frequently found in sands 
and gravels with as sharp angles and brilliant lustre as in any eruptive rock. A rounding of the angles 
cannot always be taken as evidence of wear as some minerals, especially zircon and monazite, are often 
rounded in undoubted eruptives. A lacklustre aspect without evidence of alteration is the most certain 
sign of wear. The grains of the softer minerals in the residue can be appealed to in cases of doubt. 
