) 
Sir H. H. Howorth—True Horizon of the Mammoth. 358 
as regards advance or retreat. Neighbouring glaciers often furnish 
very different results in this respect, owing to the fact that steep 
glaciers anticipate in their oscillations those the beds of which are 
less inclined. To ascertain the oscillations of glaciers, it is necessary 
to fix the actual position of the ice-snout at the end of the glacier 
with the greatest accuracy. Two methods can be employed for this 
purpose, either of which may be selected according to circumstances. 
*« Paint some signs on large boulders, not too far from the end of 
the glacier, and measure their distance from it by a tape (Richter’s 
system), or build a low wall of stones of a few yards in length, 
and, say 15 to 20 inches in height, some distance from the ice-end, 
and measure this distance (Gosset’s system). It is to be recom- 
mended that the stones of these walls should also be painted. If 
the traveller himself returns after some interval—even after only 
two or three weeks—he will be able to judge of the movement of 
the glacier, and he will have laid down a basis for future observations 
by future travellers. 
“One of the results most to be desired is an exact knowledge of 
the dates : f 
I. Of the maximum extension of the ice. 
II. Of the commencement of retreat. 
Ill. Of the minimum. 
IV. Of the commencement of fresh increase. 
“Tn dealing with a mountain group, therefore, the traveller 
should note (where he can get the information as to the past) the 
date of the commencement of the actual movement of each glacier, 
and in all cases whether the ice is in advance, or retreat, or stationary. 
Of course the rate of forward movement, or velocity of the ice, and 
the oscillations in the extension of the ice must be kept carefully 
distinct. 
“Should time and circumstances permit, a series of observations 
of the velocity of the ice is of value. These may be made after 
Tyndall’s method, by planting a line of sticks across the glacier, 
or by painting marks on boulders, the position of which relatively 
to ascertained points on the mountain-side has been accurately fixed. 
The size of the glacier, that is, the area of its basin and its length, 
as well as the slope of its bed above, as well as at the point 
measured, should be noted. The rate of movement of the ice 
appears to be connected both with the volume of the glacier and 
the inclination of its bed.” 
IV.—Tue True Horizon or tot Mammorn, Erc.: Beine a Repty 
to Mr. Marx Stirrup, F.G.S. 
By Sir Henry H. Howorrtn, K.C.I.E., M.P., F.R.S., F.G.S. 
HE very great importance of the issues involved if we are to 
make any definite progress in the study of the later deposits 
must be my excuse for this reply to Mr. Stirrup’s letter. He says 
that the only available material for discussing the results of the 
expedition of Toll and Bunge to the Siberian islands is an 
DECADE IIJ.—VOL. K.—NO. VIII. 23 
