Feb. 16, 1871] 

The overlap and irregular occurrence of the Tertiary on varions 
parts * of the Cretaceous deposits, the immense banks of flints 
containing Cretaceous fossils in the Tertiary beds, foint to an 
enormous amount of change and denudation between the consoli- 
dation of the Cretaceous and accumulation of the Tertiary deposits. 
This is accompanied by an almost entire break in the higher forms 
of life.+ It is true that the researches of Dr. Carpenter and his 
colleagues have brought to light many forms which have survived 
from the Cretaceous to our own time ; but these discoveries are 
only of the same kind as the discovery in recent times of the 
genus Lingula, or of forms allied to Encrinites. When we trace 
back to a remote antiquity ferns and other plants not very unlike 
those of our own day, Crustacea differing but little from our King 
Crab, Paludinas hardly distinguishable from recent forms—that 
does not throw doubt upon the useful grouping of the rocks from 
carboniferous to recent times. 
Species are continually being found common to two beds 
known to be separated by enormous intervals of time. Upon 
this fact Barrande founded his theory of the Colonies. But the 
classification into Mesozoic and Tertiary depends upon evidence 
that cannot be shaken by the discovery of a few more forms 
common to the two. The wonder always was that the break in 
life was so complete as it appeared to be at the close of the 
Cretaceous period, and the deep-sea dredging expeditions con- 
firm what was a friori almost a necessary inference, that deep-sea 
conditions prevailed somewhere during the whole of the period 
from the Cretaceous age to our own, and that some forms of life 
have not been destroyed or developed into anything else during 
that period ; but that is a very different thing from saying that 
there is not sufficient reason for holding that the base of the 
Tertiaries marks the commencement of a new epoch. 
T. M‘K. HuGues 

Insulation of St. Michael’s Mount, Cornwall 
TIAVING read Mr. Peacock’s letter in your publication of the 
2nd inst., I beg, through the same medium, to show that his 
reasons for supposing “‘ the mount could not have been an island 
in 1086” are groundless. 
He begins by giving the measurement of ‘‘ Domesday Book,” 
date 1086, of ‘‘ the Land of St. Michael,” and afterwards writes 
as follows :— 
“*There is an entire absence in ‘Domesday Book’ of any 
mention of island or islands on any of the coasts of Comwall. . .. 
In short, the mount could not have been an island in 1086, be- 
cause it contained at least eight times as much land as it does at 
present, probably connecting it with the main land, from which 
it is even now only one-third of a mile distant. . . . The present 
area of the mount is only thirty acres, so that there are 210 acres 
missing . . . since 1086; and in 1099, thirteen years after, we 
have a record of a catastrophe which would fully account for the 
loss’ —that being the great irruption of the sea in 1099, as re- 
corded in the Anglo-Saxon Chronicle. 
When yonr correspondent quoted the measurement of ‘‘ The 
Land of St. Michael” above referred to, he evidently imagined 
St. Michael’s Mount, with the Church or Monastery on its 
summit, to have been like a nobleman’s seat in the midst of a 
large park, with the sea at a great distance from the centre— 
and all this to have been comprehended in ‘‘The Land of St. 
Michael.” The fact, however, is that in 1086 the Mount was, 
as it still is, a rock about five furlongs in circumference at its 
base, and insulated by every tide, whilst the two parishes on 
the mainland nearest to it—viz., those of St Hilary and Perran- 
Uthnoe (which may be identical with *‘ The Land of St. Michael”), 
were then holden by the Church or Monastery of the Mount.” 
As the mount, however, is now almost universally allowed to be 
the Zktix of Diodorus Siculus, we may be sure that it was long 
before the commencement of the Christian era insulated daily as it 
is at present. I have written very fully on this subject in my work 
already referred to, published in 1862, and also ina paper printed in 
the Transactions of the Plymouth Institution for 1867-68 (pp. 17- 
37), in both of which I have exposed the error of all the trans- 
lators of Diodorus in calling the mount /é/zs instead of 7ktin, and 
have also shown that the Mount, which was called in the Cornish 
language Bre-tin (“Tin-Mount”) as well as /k-tin (“ Tin- 
* See Lyell, Student's Elements of Geol., pp. 258, 261, where attention is 
called to higher cretaceous beds than those on which the Tertiaries rest in 
eae bn s 
+ Lye . cit. p. 25 i b ) et 4 
t See seg Picante End District—its Antiquities and Natural History,” 
p. 166, 
NATURE 



399 
a ee 
Port”), has given its name, not only to Afount’s Bay, but pro- 
bably also to the whole of Britain. R. EDMONDS 
Plymouth, February 
P.S.—I had written the above before I saw Mr. H. Michell 
Whitley’s letter in your last number, which states that instead of 
“ Keiwal holds the Church of St. Michael,” as Mr. Peacock has 
translated the passage in Domesday Book (p. 2), it should have 
been ‘‘ Zhe Church of St. Michael holds Keiwal” (or Treuthal, as 
itis also called on p. 11), which is the name of a manor in the parish 
of St. Hilary. This confirms what I have above written, 
although I have adopted a different way of disproving Mr, 
Peacock’s theory. 


Aurora Borealis 
A FINE Aurora was seen last night, or this morning, from 1 to 
3A.M. It first appeared as a ¢vansverse band from N.E. to S.W., 
and passed in that course far South of the Zenith, or between Arc- 
turusand Mars. Subsequently it spread laterally and upwards ; 
presently radiated from near the Zenith to all azimuths ; and at 
2.30 A.M. some of the rays N.E. were strongly pink. 
In the spectroscope, the usual green line was gloriously bright. 
I saw it first, with a hand spectroscope, in the darkened light of 
the rough glass panels of a stairdoor. There were also faint lines 
more refrangible over the regions of E, b, and F. Rather to my 
astonishment, I was totally unable to see a red line, even when 
looking at rays abundantly pink to the naked eye. This was 
a disappointment, to say the least of it, because I had prepared, 
and had in the lower part of the field of view, red chemical lines 
to compare with anything red that should appear in the Aurora ; 
and I had seen the red line perfectly well in the fine auroras of 
last autumn, but then I had no such checks on its place, 
However, my spectroscope is still a very rough, home-made 
affair ; and I am living in hopes of something better when Go- 
vernment supplies this Observatory at last with its long-desired, 
long-delayed equatorial. 
Royal Observatory, Edinburgh, Feb. 13 Gaps 


THE THEORY OF GLACIAL MOTION* 
R. CROLL'S papers on Ocean Currents are a power- 
ful application of the modern theory of heat and 
force, to show the fallacy of Captain Maury’s explanation 
of the causes of oceanic circulation. They also discuss 
other matters of great interest, but as the concluding part 
is not yet published, we shall say no more about them 
at present, but that they well deserve careful study. 
The other paper is a criticism of the Rev. Canon 
Moseley’s supposed proof that glaciers do not descend by 
the force of gravity, and of the arguments of Messrs. 
Ball and Matthews on the other side. It will be remem- 
bered, that Canon Moseley determined by experiment the 
“shearing ” force of ice, that is, the force required to 
fracture it by parallel pressure. A plug of ice of known 
cross-section is fitted into a hole through two smooth 
boards, and the force required to break the ice by sliding 
the boards over each other is the “shearing” force. 
Increasing this in proportion to the dimensions of a 
glacier, or of any large portion of one, it was calculated 
that the force required to cause the different parts of a 
glacier to slide over each other (as they must do in 
descending a valley of constantly varying form and size) 
was at least thirty times greater than the force of gravity 
on aslope suchas glaciers easily descend. Canon Moseley 
came to the conclusion that expansion and contraction of 
the ice by heat and cold was the moving power ; and the 
fact that the glaciers move slower by night than by day, 
and in winter than in summer, was supposed to prove 
conclusively that heat is the cause of motion. 
Mr. Croll believes that Canon Moseley has demonstrated 
that gravity alone does not cause glaciers to descend, but 
he completely demolishes the theory of contraction and 
expansion. He admits that heat aids the motion, but 
maintains that it does so by acting on the molecules of 
* “On Ocean Currents.” By James Croll, of the Geological Survey of 
Scotland (3 parts). ‘On the Cause of the Motion of Glaciers.” By the 
same author (Extracted from the Pésissophical Magazine of 1870.) 
