NOVEMBER 10, 1899.] 
current, and that this decrease becomes greater 
as the frequency becomes higher. There isa 
marked analogy between this phenomenon and 
the decrease in the magnetic permeability in an 
alternating magnetic field as the frequency is 
increased, a fact which has been recently estab- 
lished by Niethammer and M. Wien. 
Wn. H. Hopss. 
DISCUSSION AND CORRESPONDENCE. 
GEOLOGICAL TIME. 
EDITOR OF SCIENCE: Sir Archibald Geikie’s 
recent forcible plea to working geologists for the 
more careful accumulation of data which may 
yield reliable estimates of geological time, 
makes the interesting suggestion given in 
SCIENCE, October 27th, by Professor Wilbur C. 
Knight, under the title of ‘ Some New Data for 
Converting Geological Time Into Years,’ seem 
very timely. The opportunities for making 
such calculations of the rate of retreat of cliffs 
under the action of subeerial decay, by employ- 
ing slow-growing trees on the escarpments as a 
chronometer, are far wider spread than at first 
thought might seem likely. 
In justice to the maiden work of a now emi- 
nent American geologist, it is proper to recall 
the fact that the first suggestion of this method 
and its first practical application were made by 
Dr. G. K. Gilbert, in 1866, when temporarily con- 
nected with the staff of the New York State 
Museum. After the excavation of the mastodon 
skeleton now standing in the State Museum, 
from a glacial pot hole in the valley of the 
-Mohawk river at Cohoes, N. Y., Mr. Gilbert 
gave attention to an estimate of the rate of re- 
treat of the cliffs of the river gorge, basing his 
observations on the degree to which the roots 
of the red cedars on the banks had been ex- 
posed by the falling away of the rock face. Mr. 
Gilbert’s observations and deductions were pub- 
lished in the 21st annual report on the New 
York State Cabinet of Natural History (1871), 
and I quote from them the following paragraph : 
“Climbing from below or lowered by a rope 
from above, I have examined nearly all these 
trees and measured in each case the circumfer- 
ence of trunk and length of exposed root. I 
_ have also counted the rings of annual accretion 
SCIENCE. 
695 
of several sections to ascertain the relation of 
size to age. From these data an idea may be 
obtained of the rate of recession of the cliff. 
The growth is exceedingly slow. <A branch of 
one and one-eighth inch in diameter showed 
100 rings of growth, and an average of six such 
branches gave 72 years per inch of diameter. 
The figures used below were obtained from two 
sections of trunks. One of these measures 
194 inches in circumference and exhibits 310 
rings ; the other gave 11 inches and 270 rings. 
In these an inch of circumference represents 
19.1 years, and an inch of diameter, 60 years.”’ 
He then gives a tabulation of results derived 
from 19 of these ancient gnarled cedars and by 
dividing the average measured length of exposed 
root by the average estimated age of the tree, 
arrives at the figure 15.2 inches as the rate of re- 
treat of the rock face per century. This figure 
for other considerations he reduced to 12 inches 
per century and upon this calculation bases his 
final statement : ‘‘ This gives as the time neces- 
sary to have removed the banks below the fall 
[Cohoes] from the deep channel to their pres- 
ent position, 35,000 years, which period I con- 
sider a minimum for the time that has elapsed 
since Cohoes falls were opposite the mastodon 
pot hole.”’ 
Twenty years ago the writer applied the 
same method to a calculation of the rate of re- 
treat of the shale escarpments along Canan- 
daigua Lake, N. Y., where these ancient cedars 
were at, that time abundant, and had the satis- 
faction of arriving at a conclusion very like 
that obtained by Mr. Gilbert. Just where 
the weakness in such calculations may lie is not 
at once evident unless there be one in admit- 
ting the annual value of the growth rings in 
the tree. Mr. Gilbert’s method, now revived 
by Professor Knight, merits renewed and gen- 
eral application. Employed with caution and 
care to exclude diverse agencies of retreat, it 
ought to afford eventually, important conclu- 
sions. JoHN M. CLARKE. 
ALBANY, N. Y. 
NEWSPAPER SCIENCE. 
To THE EDITOR OF SCIENCE: So much has 
been published far and wide this last summer 
about my intention ‘to scientifically demon- 
