Sept. 28, 1871] 
NATURE 
423 

or the atmosphere, crumbled down.” The materials thus 
supplied went to form the superficial deposits, it being 
supposed that almost the whole of Scotland was under 
the sea at the time these changes took place. We feel 
sure that Mr. Milne Home will get few geologists to agree 
with him in these conclusions. In the first place, it may 
very well be doubted whether the faults which cut the 
strata ever actually showed at the surface in the manner 
supposed. It is much more probable that the dislocations 
took place so gradually that any inequalities arising there- 
from were planed away by denudation as fast as they ap- 
peared. But even were this not the case, it is quite certain 
that the faults referred to by Mr. Milne Home must date 
back to a vastly more remote antiquity than the later 
Tertiary periods. The Scottish Coal-fields, indeed, would 
appear to be traversed by some faults which, according to 
the Geological Survey’s map and description of the South 
Ayrshire Coal-fields, do not influence the overlying Per- 
mian. It is also indisputable that the igneous dykes, 
which Professor Geikie has shown to be of Miocene age, 
are all posterior in date to the faults which shift the Coal- 
measures. Mr. Milne Home does not take into considera- 
tion the prodigious amount of denudationthat the palaeozoic 
strata of the valley of the Forth must have undergone in the 
long ages that intervened between the close of the Car- 
boniferous period and the advent of the glacial epoch. 
There cannot be any reasonable doubt that the valley of 
the Estuary of the Forth existed as a valley long before 
the dawn of the age of ice. But Mr. Milne Home’s 
memoir is taken up chiefly with the history of the drift 
deposits, which he describes in considerable detail. 
Especially valuable are the numerous sections given, and 
the long lists of localities where glacial-strize, erratic blocks, 
kaims, and the other phenomena of the drift, may be 
studied. The author inclines to the iceberg theory of the 
formation of the boulder-clay, and thinks it may have 
originated at a time when “the ocean over and around 
Scotland was full of icebergs and shore-ice, which spread 
fragments of rocks over the sea-bottom, and often stranded 
on the sea-bottom, ploughing through beds of mud, sand, 
and gravel, and blocks of stone, and mixing them together 
in such a way as to form the boulder-clay.” Mr. Milne 
Home points to the presence of beds of sand included in 
the boulder-clay as one of several objections to the land- 
ice origin of that peculiar deposit. He thinks that if the 
iceberg theory be adopted, the explanation would be 
simply this, “that icebergs came at different periods, new 
sea-bottoms being formed in the intervals.” But, on the 
other hand, if the glacier theory be accepted, then it would 
have to be admitted that the land must have sunk under 
the sea for every bed of sand we find in the boulder-clay. 
The author, however, does not seem to be aware that 
fresh-water beds are found interstratified with the boulder- 
clay, so that the difficulty in either case is equal. We 
have not space to notice several other interesting points 
treated of in this memoir, which contains so many im- 
portant data, that we can recommend it confidently to 
our geological readers. We may dispute some of the 
author’s conclusions, but it matters not what interpreta- 
tion may eventually be put upon the facts, many of the 
facts are here, and Mr. Milne Home has done good service 
in bringing them together. je Ge 


LETTERS TO THE EDITOR 
[Zhe Editor does not hold himself responsible for opinions expressed 
by his Correspondents. No notice is taken of anonymous 
communications. | 
Phenomena of Contact 
In NATURE for August 24, Mr, Stone controverts two pro- 
Positions incidentally put forward in a review of Mr, Proctor’s 
book, ‘*The Sun.” They are :— 
sits : 
<) + 


‘*y, That the irregular phenomena of internal contact of a 
planet with the sun, variously described as ‘ distortions,’ ‘ black 
drops,’ ‘ligaments,’ &c., are not always present, but are only 
seen ‘ sometimes.’ 
‘*>, That when seen they are due to insufficient optical power 
or bad definition.” 
In writing that review, I tried to avoid the assertion of any 
proposition I could not fully sustain, and therefore very willingly 
give the evidence on which these propositions rest. At the out- 
set, however, I beg leave to call especial attention to the fact 
that I did not assert the second in an absolute manner, but only 
said that it was ‘‘indicated” by observations and experiments. 
The first proposition is sustained by the fact that at the last 
transit of Mercury, the majority of those observers who have 
described the phenomena saw neither ligament nor distortion, 
but only the geometrical phenomena of contact, the planet 
preserving its rotundity to the last. 
The following is a statistical summary of the evidence on both 
sides :—Among the numerous English observations published in 
the monthly notices, fourteen describe the phenomena. Of these 
three saw the phenomena go on regularly, while eleven saw liga- 
ment, black drop, or distortion either before or after the contact. 
Among these eleven there is little agreement as to the exact 
nature of the distortion. Owing to the low altitude of the sun 
in England, I take it that the atmosphere was much less favour- 
able than on the Continent. 
At Marseilles Le Verrier saw the black drop. He used a 
seven-inch glass, of which both the centre and circumference 
were covered by a screen, which is sufficient to account for the 
phenomenon by the diffraction thus produced. Mr. Stephen, 
who observed at the same place with a very large reflector, 
‘*déclare n’avoir rien vu de pareil.” * 
Of the five observers at the Paris Observatory, Le Verrier 
sayst :—‘‘ Les observateurs ont remarque qu il ne s'est rien 
présenté de particulier, ni au moment du contact intérieur, ni 
apres ce contact. Mercure a touché le bord du Soleil en amin- 
cessant progressivement le filet de lumiére, mais sans produire le 
phénomene de la goutte.” Le Verrier was, therefore, so far as 
we know, the only observer in France who saw the black drop. 
At Madrid Ventosa may have seen several black drops ‘‘ tout- 
a-coup.” His description, however, is rather obscure. $ 
At Lund the egress was observed by Duner under very fa sour- 
able circumstances with a nine-inch glass. He says § :—‘‘ Die 
Bilder waren sehr ruhig, und die innere Beriihrung geschah in der 
Weise, dass der Lichtfaden Zwischen den Randern des Mercurs 
und der Sonne erst dann brach als seine Breite verschwindend 
klein gewordenwar. Eszeigte keine Spur einer Verdrehung der 
Bilder oder des von anderen Beobachtern erwahnten schwarzen 
Tropfens.” 
At Pulkowa fourteen observers observed the egress. I learn 
that not one saw anything but the geometrical phenomena of 
contact. 
To avoid a tedious collation of accounts which nearly all say 
the same thing, I remark that only two observers on the Continent 
saw any abnormal phenomena, namely, Kaiser at Leiden, and 
Oppolzer at Vienna. The first saw an elongation of the planet, 
which he thought might be due to maladjustment of his instru- 
ment.|| The second saw the sun’s limb pushed out by that of 
Mercury, so that apparent contact took place before the breaking 
of the thread of light.** 
Summing up all the accounts, I find the result to be :— 
Total number of observers who describe phenomena . 39 
Number who saw the planet remain perfectly round, 
and the phenomena of contact occur with entire 
regularity, and without distortion, ligament, ordrop 24 
Number who saw ligament, distortion, one or more 
drops, or other abnormal phenomena ewe OLS 
The twenty or thirty observers who do not describe the pheno- 
mena probably saw nothing abnormal, but they are not counted 
in the above list. 
The first proposition is, I conceive, fully established by the 
statistical facts cited. 
Passing now to the second, it may be remarked that when 
different observers give different descriptions of the same 
* Comptes Rendus, 1868, ii., p. 921-924. 
+ Ibid. p. 948. fs 
t Astronomische Nachrichten, vol. Ixxii., p. 356. 
§ Ibid, p. 378. 
|| Ibid, vol. lxxiii., p. 214. 
** Tbid, vol. Ixxii., p. 347. 
