3382 Correspondence—Prof. T. G. Bonney—MUr. J. Greenwood. 
THE TERM ‘‘SCHIST.” 
Srr,—Undoubtedly Mr. Rutley is right in his etymology of the 
word “schist,” and can produce authorities for the sense in which 
he proposes to use it; but notwithstanding this, and Mr. Allport’s 
support, I must confess myself a schismatie. 
But as authority has been named, I will also quote one by way of 
adding to the confusion, “It would be well to describe no structure 
as slaty or fissile except cases of transverse cleavage” (Sedgwick, 
Structure of Large Mineral Masses, Tr. G.S. ser. 2, vol. ili. p. 480). 
Mr. Rutley says (p. 289), “I use schistose and fissile as convertible 
terms when the fission is not of that perfect kind which character- 
izes slates and shales.” I confess, indeed, that I can see no reason 
etymological or otherwise why, if the term ‘‘schist” is to be extended 
beyond metamorphic rocks, those with slaty cleavage should be 
excluded. 
“The correct application” of these terms must, no doubt, to some 
extent ‘“‘depend on usage,”’ but then we must be satisfied that usage 
has not proceeded from inaccuracy and will not give rise to incon- 
venience. Surely it is always lawful to fix the sense of a term which 
previously has been rather vague. This Prof. Jukes attempted to do, 
as I venture to think, wisely, well, and intelligibly. Schists, then, in 
his sense (and I believe it one very commonly in use among geolo- 
gists) are metamorphic rocks with a fissile structure due to the 
arrangement of their mineral constituents. Schistose rocks are those 
which possess this property or seem to possess it; while slates have 
the property of cleavage; shales of lamination. Thus all schists are 
in some sense foliated rocks, though it is possible a foliated rock (as 
sometimes happens with gneiss) may not be strictly a schist. With 
such a limitation of the term we know what a writer means. Just 
think of the confusion which is caused when, on meeting with the 
word schist, we are uncertain whether the author is speaking of a 
rock that has undergone great chemical alteration or practically none 
at all. Seeing then that (as it seems to me) Prof. Jukes’s limitation 
supplies us with a term which we really want, I trust that no attempt 
will be made to extend the name “schist” beyond the group of 
metamorphic rocks. T. G. Bonney. 
ECCENTRICITY AND GLACIAL EPOCHS. 
Str,—There are some points in Mr. Hill’s remarks as to the causes 
of a Glacial Epoch which I think require rectifying. He admits 
that the total amount of heat radiated by the sun and received by 
any portion of the earth in any one year is a fixed amount. In his 
letter in your issue of April, he admits as probable, that the heat 
given up in the formation of snow, being disengaged in the upper 
regions produces little effect at the ground. Now, granting these, 
I think it is clearly demonstrable that a Glacial Epoch may be 
caused by an increased snowfall. For say in any region we have, 
through increased eccentricity a fall of two feet of snow as against 
a fall of one foot before such increase of eccentricity. Then, with 
