167 
As these clefts were washed clean, this supply would naturally 
fail.* 
This limestone is mineralogically identified with the rock at 
Oreston, which furnished the materials for the Plymouth 
breakwater. 
Mr. Pengelly asserts that he u has always abstained from, 
and cautioned others against insisting that the thickness of 
the stalagmite is a perfectly trustworthy chronometer ; never- 
theless, it seems fair to ask those who deny that it is of any 
value, to state the basis of their denial.” 
This challenge I shall accept ; but in the mean time must ask 
the reader to note that Mr. Pengelly passes on immediately to 
say that “ such estimates, if sufficiently multiplied, ai*e of great 
value. ” 
Now it may be conceded that, under some circumstances, the 
growth of stalagmite may be shown to be so far continuous as 
presumably to indicate a certain lapse of time. The observa- 
tions of Mr. Boyd Dawkins, f on the rate at which stalagmite 
is being accumulated in the Ingleborough Cave, are admitted 
to be of this character. “ The author states, on what appears 
to be most satisfactory evidence, that the apex of a boss of 
stalagmite known as the Jockey’s Cap, in that cave, rising 
from the crystalline pavement to a height of 2‘50 feet, was 
found, by careful measurement, on March 13th, 1873, to be 
87 inches from the roof ; whilst when measured by James 
Farrar, on October 30th, 1845, it was 95 - 25 inches from 
it; so that the upward growth has been 8'25 inches in 27‘37 
years ; giving an average vertical growth of ‘3 inch per year. 5 ’ 
“ On the strength of this fact,” the author remarks that, 
“ all the stalagmites and stalactites in the Ingleborough Cave 
may not date further back than the time of Edward III., if the 
‘ Jockey’s Cap’ be taken as a measure of the rate of deposition.” 
“ It is evident,” he continues, “ from this instance of rapid 
accumulation, that the value of a layer of stalagmite, in fixing 
the high antiquity of deposit below it, is comparatively little. 
The layers, for instance, in Kent’s Hole, which are generally 
believed to have demanded a considerable lapse of time, may 
possibly have been formed at the rate of a quarter of an inch 
per annum /” At this rate “ twenty feet of stalagmite might 
be formed in 1,000 years ” (p. 41). 
We have in the above carefully-recorded experiment an 
approach to the accuracy of a chronometer in a calculation 
derived from the increment of real stalagmite ; but it will be 
* McEnery, p. 259. 
f Cave-Hunting, W. Boyd Dawkins, F.R.S., pp. 39, 40, and Appendix II. 
